fix(cache): render non-Response results the way FastAPI does

[allen0099]

Closes #99.

Problem

When a @cache handler returned plain data, get_response() built the response with response_class(content=result) and handed FastAPI a finished Response, so FastAPI skipped its own handling of the return value:

• A Pydantic model, datetime or UUID failed to encode (500), and response_model filtering never ran.
• @app.get(..., status_code=202) answered 200.
• Status and headers set on an injected response: Response were lost.

Change

In get_response(), for non-Response results:

• Response model: new _serialize_result(). When the route has a response model (declared, or inferred from the return annotation), the result is validated with a pydantic.TypeAdapter, using from_attributes=True so a different model such as UserIn → UserOut works. It is then dumped with mode="json" and the route's response_model_include/exclude/by_alias/exclude_unset/exclude_defaults/exclude_none. Otherwise the result goes through jsonable_encoder.
• Adapter reuse: APIRoute is unhashable, so the adapter is stored once per route as an attribute instead of being kept in a dict.
• Status code: the route's status_code applies, overridden by the injected sub-response's status when the handler set one. This matches FastAPI's _build_response_args. A status that allows no body (for example 204) gets an empty body.
• Headers: the sub-response is the Response instance among the handler's kwargs, and its raw headers are appended.

Cache entries already store the status code and headers, so hits replay them. Set-Cookie is still never stored.

I used public APIs (TypeAdapter, jsonable_encoder, fastapi.utils.is_body_allowed_for_status_code) rather than calling fastapi.routing.serialize_response. Its signature changes between FastAPI releases, and the dependency is unpinned.

Tests

The new tests/test_cache_rendering.py covers:

• non-JSON types with and without a response model, on both miss and hit;
• response-model filtering with response_model_exclude_none;
• the route's status_code on miss and hit;
• a 204 with an empty body;
• an injected response's header and status on miss and hit.

All 5 fail against the old get_response().

Checks

• ruff check / format on fastapi_cachex, tests and scripts
• mypy strict on the package; mypy on tests and scripts
• pytest against live Redis and Memcached (CACHEX_REQUIRE_LIVE_SERVERS=1): 725 passed, 100% coverage
• zensical build --strict

docs/HTTP_CACHING.md gains a paragraph on how plain return values are rendered. The CHANGELOG entry is at the top of ### Fixed, a spot the open PRs #132 and #133 don't touch.