[fbreceiver] Ignore cgo DNS goroutines in goleak leak checker on macOS

[macdewee]

Proposed commit message

WHAT: Add goleak.IgnoreAnyFunction("net.cgoResSearch") to the skip list in oteltest.VerifyNoLeaks. This is the same pattern already used for net.(*netFD).connect and net/http.(*Transport).startDialConnForLocked.

WHY: TestLeak, TestNewReceiver, and TestMultipleReceivers in x-pack/filebeat/fbreceiver fail intermittently on macos-15-intel CI runners. The goroutine leak checker detects a goroutine stuck for ~1 minute in a cgo DNS res_nsearch syscall (net.cgoResSearch → syscall.syscalln). This is a networking/infrastructure issue on the runner where a DNS lookup issued during the test never returns — not a real leak in production code. The filter is narrow and targets only that specific cgo DNS frame.

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. (test infrastructure change; the leak only manifests on macOS CI)
• I have added an entry in ./changelog/fragments using the changelog tool. (test-only change)

Disruptive User Impact

None. Test infrastructure change only.

How to test this PR locally

The DNS hang only occurs on restricted macOS CI networking and is not reliably reproducible locally. Verify that the existing fbreceiver tests still pass:

cd x-pack/filebeat
go test ./fbreceiver/... -v -count=1

Related issues

• Relates Allow overriding beat hostname via ELASTIC_AGENT_HOSTNAME env var #52126 (observed failing in CI against this unrelated PR)

Logs

=== FAIL: x-pack/filebeat/fbreceiver TestLeak/unhealthy_consumer (6.73s)
    oteltest.go:251: found unexpected goroutines:
        [Goroutine 54 in state syscall, 1 minutes, with syscall.syscalln on top of the stack:
        syscall.syscalln(...)
        internal/syscall/unix.ResNsearch(...)
        net._C_res_nsearch(...)
        net.cgoResSearch(...)
        net.resSearch.func1()
        net.doBlockingWithCtx[...].func1()
        ]

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[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @macdewee? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
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To fixup this pull request, you need to add the backport labels for the needed
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• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
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