Fix(target-allocator): accept prometheus receiver with only target_allocator block

Closes #2998

Summary

When an OpenTelemetryCollector CR has a prometheus receiver containing only a target_allocator: block (no config: / scrape_configs:), reconciliation fails with "no prometheusConfig available as part of the configuration". this is wrong - that shape is a legitimate "TA-only" mode where the target allocator supplies scrape configuration externally (typically from discovered PrometheusCR objects, or a configmap mounted on the TA, outside the operator's surface).

Note: a 2025-09-17 comment on the issue claimed this can't be reproduced, but I can't confirm that — the bug report shape (receiver with target_allocator: only, no config: at all) still triggers the error on main. unit-level repro below.

What Changed

• internal/manifests/targetallocator/adapters/config_to_prom_config.go - AddTAConfigToPromConfig now skips the scrape_configs strip when config: is absent instead of erroring; when config: is present the existing strip still runs. ValidateTargetAllocatorConfig returns nil when config: is absent (TA-only mode signal), instead of cascading the missing-config error from GetScrapeConfigsFromPromConfig.
• internal/manifests/targetallocator/adapters/config_to_prom_config_test.go - the previously-asserts-bug sub-case ("missing config property" → expects the old error) is removed from the negative test, and two positive sub-cases are added: "TA-only mode: no config block, only target_allocator set" and "TA-only mode: no config block, no target_allocator block". both assert no spurious config: key is inserted on the way out. TestValidateTargetAllocatorConfig gets its "receiver config empty and PrometheusCR disabled" row flipped from "expect error" to "expect nil", plus a new explicit "no config block but target_allocator set, PrometheusCR disabled" row.
• tests/e2e-targetallocator/targetallocator-no-promconfig/ - new chainsaw e2e: deploys an OpenTelemetryCollector with receivers: { prometheus: {} } and targetAllocator.enabled: true + prometheusCR.enabled: true, then asserts the collector and TA come up Ready and that the rendered collector configmap contains the operator-injected target_allocator block under prometheus: with no config: key.
• .chloggen/fix_target-allocator-only-config.yaml - bug_fix entry.

Reproduction

a throwaway test (not committed) calls both functions with the exact config from the bug report. at HEAD^ both fail with the bug's error; at HEAD both pass:

=== BEFORE (HEAD^): test against the initial source ===
=== RUN   TestRepro2998
    repro2998_test.go:32: 
        	Error Trace:	.../repro2998_test.go:32
        	Error:      	Received unexpected error:
        	            	no prometheusConfig available as part of the configuration
        	Test:       	TestRepro2998
        	Messages:   	AddTAConfigToPromConfig must accept TA-only config
    repro2998_test.go:35: 
        	Error Trace:	.../repro2998_test.go:35
        	Error:      	Received unexpected error:
        	            	no prometheusConfig available as part of the configuration
        	Test:       	TestRepro2998
        	Messages:   	ValidateTargetAllocatorConfig must accept TA-only config when PrometheusCR is disabled
--- FAIL: TestRepro2998 (0.00s)
FAIL
FAIL	github.com/open-telemetry/opentelemetry-operator/internal/manifests/targetallocator/adapters	0.011s

=== AFTER (HEAD): test against the PR source ===
=== RUN   TestRepro2998
--- PASS: TestRepro2998 (0.00s)
PASS
ok  	github.com/open-telemetry/opentelemetry-operator/internal/manifests/targetallocator/adapters	0.006s

The regression coverage that lands in this PR is in config_to_prom_config_test.go and the new chainsaw test under tests/e2e-targetallocator/targetallocator-no-promconfig/; the throwaway above is just to demonstrate the before-state in a single test file at the same commit.

Design

• skip rather than synthesise: the earlier draft of this fix synthesised an empty config: {} map so downstream code could keep writing into it. swiatekm pointed out the prometheus receiver doesn't require a config: block at all, so the synthesis is unnecessary. the current diff just guards the delete(prometheusCfg, "scrape_configs") behind the presence check — if there's no config:, there are no scrape_configs to strip and nothing else in AddTAConfigToPromConfig touches prometheusCfg. the rendered collector config stays minimal (just the operator-injected target_allocator block) and the chainsaw assert pins that shape.
• validator returns nil on missing config: not a permissive blanket-bypass — when config: is absent and TA is enabled (this whole code path runs only when TA is enabled), there is genuinely nothing to validate locally. the TA owns its scrape-config source.
• target_allocator.endpoint user-supplied value is still overwritten by the operator (unchanged). that's existing behavior; this PR doesn't change endpoint semantics, only the validation/strip path.

Tests

$ go test ./internal/manifests/targetallocator/... -count=1
ok  	github.com/open-telemetry/opentelemetry-operator/internal/manifests/targetallocator	0.055s
ok  	github.com/open-telemetry/opentelemetry-operator/internal/manifests/targetallocator/adapters	0.009s

$ go test ./internal/manifests/... -count=1 -short
ok  	github.com/open-telemetry/opentelemetry-operator/internal/manifests	0.035s
ok  	github.com/open-telemetry/opentelemetry-operator/internal/manifests/collector	0.064s
ok  	github.com/open-telemetry/opentelemetry-operator/internal/manifests/collector/adapters	0.006s
ok  	github.com/open-telemetry/opentelemetry-operator/internal/manifests/manifestutils	0.020s
ok  	github.com/open-telemetry/opentelemetry-operator/internal/manifests/opampbridge	0.027s
ok  	github.com/open-telemetry/opentelemetry-operator/internal/manifests/targetallocator	0.051s
ok  	github.com/open-telemetry/opentelemetry-operator/internal/manifests/targetallocator/adapters	0.010s

$ go test ./internal/webhook/... -count=1 -short
ok  	github.com/open-telemetry/opentelemetry-operator/internal/webhook	9.847s
ok  	github.com/open-telemetry/opentelemetry-operator/internal/webhook/podmutation	9.857s

$ gofmt -l internal/manifests/targetallocator/adapters/
(no output - clean)

$ go vet ./internal/manifests/targetallocator/... ./internal/webhook/...
(no output - clean)

make precommit not run end-to-end locally (envtest binaries download is slow on first run); the targeted go test runs above cover the same packages. the new chainsaw test runs under the operator's e2e-targetallocator suite in CI.

Notes

• no API changes, so make ensure-update-is-noop should be a no-op - verified by not editing anything under apis/.
• happy to split the ValidateTargetAllocatorConfig change out into a separate PR if maintainers prefer; both pieces are needed to fully resolve Target Allocator doesn't accept only target_allocator config in the collector config #2998 since both functions are called from the webhook (collector_webhook.go:330-337).