fix: processor-level config takes precedence over top-level settings

[lazureykis]

Summary

Fixes #976.

When both traces_exporter (or bsp_* settings) and processor-level config are set, the processor-level config was silently overridden by the top-level value. For example:

application:set_env(opentelemetry, processors, [
    {otel_batch_processor, #{exporter => {my_exporter, #{}}}}
]),
application:set_env(opentelemetry, traces_exporter, {other_exporter, #{}}),

The batch processor would end up using other_exporter instead of my_exporter.

Changes

• merge_processor_config_/4: Simplified to always respect processor-level config — if a key already exists in processor opts, top-level/env values don't override it.
• transform(span_processor, batch|simple): Returns empty #{} instead of BATCH_PROCESSOR_DEFAULTS/SIMPLE_PROCESSOR_DEFAULTS. Defaults are already applied via maps:merge in merge_processor_config/4, so pre-populating was redundant and prevented the merge logic from distinguishing user-set values from defaults.
• Removed the now-unused is_default/3 helper.

New precedence order

processor opts > top-level/env config > BATCH_PROCESSOR_DEFAULTS

Tests

Added processor_exporter_not_overridden (12 assertions) and processor_config_precedence_with_os_env (4 assertions) covering:

• Processor exporter not overridden by top-level traces_exporter
• Processor none exporter preserved
• bsp_*/ssp_* settings don't override processor-level equivalents
• bsp_*/ssp_* still apply when keys are absent from processor opts
• OS env (OTEL_TRACES_EXPORTER) doesn't override processor-level exporter
• Multiple processors get independent config resolution
• Reporter's exact scenario from Batch processor app env config: user-provided exporter overridden by BATCH_PROCESSOR_DEFAULTS merge #976