REST API: Cache route maps in WP_REST_Server::get_routes()

[wppoland]

Trac ticket: https://core.trac.wordpress.org/ticket/39473

get_routes() is called multiple times per request (route matching, schema generation, OPTIONS handling). The result is deterministic between route registrations, so this PR memoizes the per-namespace map on the server instance and busts the cache whenever register_route() mutates $this->endpoints.

Tests cover:

• rest_endpoints filter fires once across repeated calls
• Cache keyed by namespace
• Cache invalidated when a new route is registered
• Cached and uncached results are identical

Note for reviewers: caching the post-rest_endpoints-filter result means filters added after the first call to get_routes() no longer take effect for that request. This matches the perf goal but is a documented filter-contract change worth weighing - happy to refactor to cache only the pre-filter map if reviewers prefer.

Use of AI Tools

AI assistance: Yes
Tool(s): Antigravity IDE
Model(s): Gemini 2.5 Pro
Used for: Code analysis, conflict resolution, test suite updates, and patch verification. All changes were manually reviewed and tested locally.

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[github-actions]

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[github-actions]

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[wppoland]

Rebased cleanly onto current trunk to resolve the conflict in WP_REST_Server::get_routes() (incorporating the trunk @phpstan-var annotation) and dropped the redundant PHPStan baseline commit. All 133 Tests_REST_Server tests pass cleanly locally with 0 PHPCS violations.