wal-protocol: extract v1 Envelope into a dedicated module

[muhamadazmy]

wal-protocol: extract v1 Envelope into a dedicated module

Summary:

Move the existing Envelope, Header, Source, Destination, and Command
types out of wal-protocol/src/lib.rs into a new v1 submodule, and
re-export them from the crate root for backward compatibility.

This is a pure refactor — no behavior or wire-format change — that
prepares the crate for the upcoming v2 envelope by carving out a
clear namespace for the legacy types.

---

Stack created with Sapling. Best reviewed with ReviewStack.

• worker: write v2 envelopes with typed commands #4700
• worker: decode partition records as v2 envelopes with v1 fallback #4696
• wal-protocol: introduce v2 Envelope with bilrost encoding and lazy payloads #4695
• -> wal-protocol: extract v1 Envelope into a dedicated module #4694

[github-actions]

Test Results

  8 files  + 1    8 suites  +1   4m 38s ⏱️ + 1m 51s
 50 tests + 3   50 ✅ + 3  0 💤 ±0  0 ❌ ±0 
218 runs  +18  218 ✅ +18  0 💤 ±0  0 ❌ ±0 

Results for commit e375998. ± Comparison against base commit b128276.

This pull request removes 4 and adds 7 tests. Note that renamed tests count towards both.

dev.restate.sdktesting.tests.KafkaIngress ‑ handleEventInCounterService(URI, int, Client)
dev.restate.sdktesting.tests.KafkaIngress ‑ handleEventInEventHandler(URI, int, Client)
dev.restate.sdktesting.tests.UpgradeWithInFlightInvocation ‑ inFlightInvocation(Client, URI)
dev.restate.sdktesting.tests.UpgradeWithNewInvocation ‑ executesNewInvocationWithLatestServiceRevisions(Client, URI)

dev.restate.sdktesting.tests.Combinators ‑ awakeableOrTimeoutUsingAwaitAny(Client)
dev.restate.sdktesting.tests.Combinators ‑ firstSuccessfulCompletedAwakeable(Client)
dev.restate.sdktesting.tests.Custom ‑ run(CustomTestConfig, URI, URI)[1]
dev.restate.sdktesting.tests.UserErrors ‑ failSeveralTimesWithMetadata(URI)
dev.restate.sdktesting.tests.UserErrors ‑ internalCallFailurePropagationWithMetadata(URI)
dev.restate.sdktesting.tests.UserErrors ‑ invokeTerminallyFailingCallWithMetadata(URI)
dev.restate.sdktesting.tests.UserErrors ‑ sideEffectWithTerminalErrorWithMetadata(URI)

♻️ This comment has been updated with latest results.