[9.4](backport #50555) fix(otel): fix beatprocessor to honor when conditions

[mergify]

Proposed commit message

The beat processor instantiated each processor by calling its New constructor directly, bypassing the conditional wrapping that processors.Namespace.Register applies on the standard libbeat path. The when field was unpacked into the config but silently dropped, so processors ran unconditionally.

Wrap each constructor with processors.NewConditional before invoking it so conditionals are honored, matching the behavior of standalone beats.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

How to test this PR locally

go test -count=1 -v -run '^TestCreateProcessor$' ./x-pack/otel/processor/beatprocessor/
go test -count=1 -tags=integration -v -run '^TestBeatProcessorWhenCondition$' ./x-pack/filebeat/tests/integration/

./script/stresstest.sh ./x-pack/otel/processor/beatprocessor '^TestCreateProcessor$' -p 4
2m10s: 17012 runs so far, 0 failures, 4 active

./script/stresstest.sh --tags integration ./x-pack/filebeat/tests/integration '^TestBeatProcessorWhenCondition$' -p 2
3m10s: 683 runs so far, 0 failures, 2 active

Related issues

• Closes beatprocessor OTel component ignores when conditions on processors #50549

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  This is an automatic backport of pull request fix(otel): fix beatprocessor to honor when conditions #50555 done by Mergify.

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