test(e2e): verify event gateway dump excludes children by default

[rspurgeon]

The filtered event-gateway dump scenario checked the parent but could miss a
regression that includes children without --include-child-resources.
Extend step 003-dump-with-filter to assert that all seven child collections
resolve to null, matching their omission from actual dump YAML. Document the
default behavior beside the step.

Fixes #2182.

Validation:

• Passed Go modernization, formatting, build, lint (serialized to avoid another
  running linter), unit tests, integration tests, installer checks, and all 96
  E2E metrics tests.
• Live fixture creation, the updated filtered dump assertions, and cleanup
  passed with KONGCTL_E2E_RESET=0 and step 002-dump-event-gateways skipped.
  The captured YAML confirms all seven child collection keys are omitted.
  Artifacts: /home/rspurgeon/go/e2e-artifacts/20260915-224719.
• Full scenario attempts passed the positive child assertions but failed at
  the unchanged round-trip plan assertion (14 creates, then 7 creates, instead
  of zero changes). Another event-gateway E2E scenario was running locally and
  the fixture disappeared between runs, suggesting shared-environment
  interference; the cause is not confirmed. A clean full-scenario pass remains
  unverified. Cleanup was run successfully afterward.
  Latest full-run artifacts:
  /home/rspurgeon/go/e2e-artifacts/20260915-224759.

[kong-api-ops]

✅ KSAI Review: Finished

1. 03:58 UTC Reading the code changes and applicable review conventions

---

Run report (federated) · 1 paid run · $0.0205 total

#	Engine	Result	Model	Turns	Cost
1	opencode	success	zai-org/GLM-5.3-Flash/high	15	$0.0204

Reviewing cost $0.0204; deciding how to run it cost $0.0001

[kong-api-ops]

Check	Result
Scope	Adds one assertion to an e2e dump scenario verifying child arrays are absent without --include-child-resources
Mandate	default-code-reviewer
Findings	None
Findings audit	Skipped (no candidate findings)

No issues found. Verified against the dump implementation and the harness matcher: the seven child fields are omitempty and only populated behind the flag, and a missing key resolves to nil, which matches the expected null. The null-expectation idiom matches existing scenarios. Additional Risk pass: no additional concerns found.