refactor(cache): build the key only where the backend is used; report media_type

[allen0099]

Summary

Closes #182
Closes #184

Both are part of #121.

#182: @cache wrapper

• The cache key is now built right before the backend read. The no-store branch and the backend-bypass branch (private, no ttl, ttl=0) never touch the backend, so they used the key only in debug logs, and a custom key_builder ran for nothing there. Those logs now show the request path.
• if not req (which relies on Request.__len__) becomes if req is None.
• The build_cache_control closure is now a module-level _build_cache_control(...) with keyword-only arguments and the same output. It is still built once per decorated route.

No behaviour change apart from when key_builder is called. It still runs exactly once per GET that reads the backend: miss, hit and 304 alike.

#184: monitoring routes

• /cached-records gains a media_type field with the stored CacheEntry.media_type, or null when there is none. content_type stays and is always "bytes" for compatibility.
• CACHE_KEY_MAX_PARTS is a maxsplit count, so it is renamed CACHE_KEY_MAX_SPLIT. The old name remains as an alias.
• HTTP_CACHING.md (EN and zh-TW) documents media_type and points readers away from content_type.

Tests

• New file tests/test_cache_internals.py:
  • key_builder is not called for no_store, private, no ttl, ttl=0 (including a revalidating request) or non-GET;
  • it is called exactly once per request on a cached route (miss, hit, 304);
  • _build_cache_control covers the directive order, stale-if-error and no-cache dropping the rest.
• tests/test_routes.py: media_type for JSON and text/plain entries, null for an entry without one, and the constant alias.
• Mutation-checked each change against the full suite:
  • building the key before no_store fails only the new key-builder tests;
  • reporting media_type=None fails only the new media-type test;
  • dropping must-revalidate under no-cache fails the new _build_cache_control case plus the existing test_no_cache_with_revalidate.
• uv run pytest passed without live servers: 813 passed and 190 skipped, where the skips are the Redis/Memcached tests. Coverage is 93.03%, with cache.py and routes.py at 100%.
• pre-commit passed on all files, and both strict docs builds (EN and zh-TW) passed.

CHANGELOG

Added

• /cached-records reports each entry's media_type. It is the media type the response was stored with, or null when it had none. content_type is still returned for compatibility but is always "bytes". CACHE_KEY_MAX_PARTS in fastapi_cachex.routes is renamed CACHE_KEY_MAX_SPLIT, since it is a maxsplit count; the old name remains as an alias. (#184)

Changed

• @cache builds the cache key only when it reads or writes the backend. A custom key_builder is no longer called for no_store, private or TTL-less routes, where the key only fed debug logs. (#182)