[8.19](backport #47969) fbreceiver: refactor receiver tests for maintainability

[orestisfl]

Proposed commit message

This is a manually resolved backport of #47969 to the 8.19 branch. The original automatic backport (#47994) had merge conflicts.

Changes:

• Refactor TestMultipleReceivers to easily scale to n>2 receivers
• Clean up *.sock files
• Add multiReceiverHelper struct to encapsulate receiver test setup
• Add helper functions: hostFromSocket, writeFile
• Use UUID for socket path generation instead of random bytes
• Use DialContext instead of Dial to fix noctx lint warning

Conflict Resolution:

The conflict was in x-pack/filebeat/fbreceiver/receiver_test.go. The 8.19 branch had updated assertions to check agent.otelcol.component.id and agent.otelcol.component.kind fields instead of message. The resolution:

• Kept the refactored loop style from fbreceiver: refactor receiver tests for maintainability #47969 (iterating over helpers)
• Adapted the 8.19-specific assertions to use the new helper pattern:

  assert.Equalf(c, "filebeatreceiver/"+helper.name, logs[helper.name][0].Flatten()["agent.otelcol.component.id"], ...)
  assert.Equalf(c, "receiver", logs[helper.name][0].Flatten()["agent.otelcol.component.kind"], ...)

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

Run the test:

cd x-pack/filebeat/fbreceiver
go test -v -count=1 -run "TestMultipleReceivers|TestNewReceiver|TestLeak" ./...

Related issues

• Backport of fbreceiver: refactor receiver tests for maintainability #47969
• Resolves conflicts in [8.19](backport #47969) fbreceiver: refactor receiver tests for maintainability #47994
• Blocks javascript processor: replace global paths with per-beat paths #47870

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