[zhaohuabing]

What this PR does / why we need it:

EndpointSlice changes currently wait for full route, policy, and xDS translation. This adds the disabled-by-default EndpointFastPath runtime flag so eligible endpoint updates can reach Envoy sooner.

• A publisher per IR key owns publication state. It combines ordered full results and IR-key deletions with endpoint updates, using the current configuration context and endpoint revisions from both paths to prevent stale updates from overwriting newer applied endpoints.
• The fast path reuses the existing endpoint assignment builder and shares unchanged resources while rebuilding and hashing changed endpoint assignments. Other xDS resources and versions remain unchanged during an endpoint-only publication.
• Full reconciliation still runs for every endpoint change. Empty/nonempty transitions, address-type changes, incompatible contexts, patched resources, and extension hooks retain full-path handling.
• Adds publication, replay, deletion, retry, concurrency, and translation tests; an opt-in e2e profile; operational documentation; and the publication design.

Which issue(s) this PR fixes:

Fixes #9777

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PR Checklist

• Authorship & ownership: Coding agents / AI assistants are welcome, but I have reviewed every change, understand how and why it works, can explain and maintain it, and take full responsibility for this PR. I have not submitted generated output I do not understand.
• DCO: All commits are signed off (git commit -s). See DCO: Sign your work.
• API agreed first: If this PR contains API changes (changes under /api), the API was discussed and agreed before the implementation. The API change can be in a separate PR, or in the same PR, but the API must be agreed before implementation. N/A if this PR does not contain API changes.
• Required checks pass: make generate gen-check, make lint, and the unit-test/coverage build pass. (Flaky e2e failures are not considered breakages, but gen-check, lint, and coverage MUST pass.)
• Tests added/updated: New/changed code is covered by appropriate tests. N/A if this PR does not contain code changes.
• Docs: User-facing changes update the docs, either in this PR or a follow-up PR. N/A if this PR does not contain user-facing changes.
• Release notes: For any non-trivial change, added a release-note fragment under release-notes/current/<section>/<pr-number>-<slug>.md (see release-notes/current/README.md for sections and naming). N/A if this PR does not contain non-trivial changes.
• Generated files committed: Ran make gen-check and committed the result if API/helm charts/modules changed.
• Scope & compatibility: The PR is reasonably scoped (no unrelated changes) and preserves backward compatibility, or any breaking change is called out above and documented in release-notes/current/breaking_changes/.
• Codex review: Requested a Codex review and addressed all of its comments.
• Copilot review: Requested a Copilot review and addressed all of its comments.

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[codecov]

Codecov Report

❌ Patch coverage is 79.63875% with 124 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.13%. Comparing base (44b1a63) to head (2c2bc6f).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/xds/runner/endpoint_publisher.go	75.43%	42 Missing ⚠️
internal/provider/kubernetes/endpoint_fast_path.go	67.67%	32 Missing ⚠️
internal/xds/cache/snapshotcache.go	0.00%	16 Missing ⚠️
internal/endpoint/source.go	93.65%	8 Missing ⚠️
internal/xds/runner/runner.go	55.55%	8 Missing ⚠️
internal/provider/kubernetes/controller.go	61.11%	7 Missing ⚠️
internal/xds/cache/endpoint_snapshot.go	81.81%	6 Missing ⚠️
internal/xds/translator/endpoint_fast_path.go	96.59%	3 Missing ⚠️
internal/gatewayapi/runner/runner.go	66.66%	1 Missing ⚠️
internal/message/watchutil.go	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10398      +/-   ##
==========================================
- Coverage   82.20%   82.13%   -0.08%     
==========================================
  Files         277      282       +5     
  Lines       44014    44571     +557     
==========================================
+ Hits        36183    36607     +424     
- Misses       7830     7963     +133     
  Partials        1        1              

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[zhaohuabing]

/retest