feat(gateway): implement annotation inheritance from Gateway to Route

[lexfrei]

Summary

Implements Solution 2 from proposal #5919: annotation inheritance from Gateway to Route resources.

• Gateway annotations serve as defaults for all attached Routes
• Route annotations override Gateway defaults when the same key is specified
• Supports all annotation types: target, ttl, hostname, controller, and provider-specific

This enables centralized configuration (e.g., default TTL, Cloudflare proxy settings) on Gateway while allowing per-Route customization when needed.

Use Case

Gateway:
  annotations:
    target: "172.16.6.6"  # intranet default
    ttl: "300"

Route1 (internal):
  # No annotations → inherits 172.16.6.6, TTL 300

Route2 (external):
  annotations:
    target: "1.2.3.4"     # override for external access
    ttl: "60"

Changes

• source/gateway.go: Added mergeAnnotations() helper and resolveResult struct
• source/gateway_test.go: Unit tests for mergeAnnotations() including mutation safety
• source/gateway_httproute_test.go: 9 integration tests covering inheritance, override, A→CNAME, multi-gateway scenarios
• docs/sources/gateway-api.md: Updated documentation with examples
• docs/annotations/annotations.md: Updated Gateway API section

Test Plan

• All existing Gateway tests pass
• New unit tests for mergeAnnotations() (9 cases + mutation test)
• New integration tests for annotation inheritance (9 cases)
• Linter passes

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @lexfrei. Thanks for your PR.

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[szuecs]

/hold likely we will not integrate HTTPRoute in the future , but ListenerSet as told by gateway-api. We should wait until they finalized the API and what external-dns should do to integrate.

[lexfrei]

@szuecs re-marked this as a draft, just a reference for #5919. We can hold this as long as you need and merge/drop at any time