feat: support cross-namespace quota backends

[PatilHrushikesh]

Description

Add cross-namespace support for AI Gateway QuotaPolicy targets.

A QuotaPolicy can now target an AIServiceBackend in another namespace by setting targetRefs[].namespace. If the namespace is omitted, it defaults to the QuotaPolicy namespace, preserving existing behavior.

The change:

• Resolves AIServiceBackend targets using fully qualified namespace/name identities.
• Lists and matches QuotaPolicy resources across namespaces when building route quota configuration.
• Propagates namespaced backend identities through controller state, xDS quota descriptors, and extproc response metadata.
• Correctly notifies affected routes when a cross-namespace QuotaPolicy is deleted.
• Updates the QuotaPolicy CRD, generated deepcopy code, API documentation, and quota-policy documentation.
• Adds and updates controller and quota rate-limit tests for namespaced targets.

Related Issues/PRs (if applicable)

None.

Special notes for reviewers (if applicable)

• Omitting targetRefs[].namespace remains backward compatible and resolves to the policy namespace.
• Debug logging and local test/deployment artifacts are intentionally excluded from this PR.

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[codecov]

Codecov Report

❌ Patch coverage is 88.73239% with 40 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/controller/quota_policy.go	81.63%	18 Missing ⚠️
internal/controller/gateway.go	83.72%	7 Missing ⚠️
internal/extensionserver/quota_ratelimit.go	92.50%	6 Missing ⚠️
internal/controller/ai_gateway_route.go	93.54%	4 Missing ⚠️
internal/controller/referencegrant_controller.go	84.00%	4 Missing ⚠️
internal/extproc/processor_impl.go	93.75%	1 Missing ⚠️

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