fix: type CachedResponse.__init__ for mypy --strict (#628)

[jbbqqf]

Summary

CachedResponse.__init__ is currently un-annotated, so mypy --strict
codebases that import and call CachedResponse(...) get a
[no-untyped-call] error even though the class is part of the public
API. Annotate the constructor (response: Response,
timeout: int | None, -> None) and document the in-place __dict__
swap so the intent is clear to reviewers.

Fixes #628.

Context

CachedResponse is exported from flask_caching and is intended to be
constructed by user view code (e.g. return CachedResponse(make_response(...), timeout=2),
as in tests/test_view.py::test_cache_timeout_dynamic). Without
annotations, mypy refuses to consider it a typed callable. The class
already inherits typing from flask.Response, and timeout is already
declared as a class attribute, so this change just exposes those types
on the constructor signature.

Changes

• src/flask_caching/__init__.py: add parameter and return annotations
  on CachedResponse.__init__, type the timeout class attribute as
  int | None, and leave a short comment explaining the in-place
  __dict__ adoption.
• tests/test_init.py: new test_cached_response_init_is_typed
  regression test that introspects inspect.signature to assert the
  constructor declares -> None and all non-self parameters carry an
  annotation; plus test_cached_response_construction_with_flask_response
  which exercises construction end-to-end.
• CHANGES.rst: add Unreleased entry.

Reproduce BEFORE/AFTER yourself (copy-paste)

# --- one-time setup ---
git clone https://github.com/pallets-eco/flask-caching.git /tmp/fc-628 && cd /tmp/fc-628
python3.11 -m venv .venv && . .venv/bin/activate
pip install -e . pytest >/dev/null

# --- BEFORE: origin/main, expect FAIL ---
git checkout origin/main
git fetch https://github.com/jbbqqf/flask-caching.git fix/628-cachedresponse-init-type
git checkout FETCH_HEAD -- tests/test_init.py
pytest tests/test_init.py::test_cached_response_init_is_typed -v
# Expected: 1 failed - AssertionError "return annotation is <class 'inspect._empty'>; expected None"

# --- AFTER: this branch, expect PASS ---
git checkout FETCH_HEAD
pytest tests/test_init.py::test_cached_response_init_is_typed -v
# Expected: 1 passed

What I ran locally

$ pytest tests/test_init.py -v
...
tests/test_init.py::test_cached_response_init_is_typed PASSED
tests/test_init.py::test_cached_response_construction_with_flask_response PASSED
======================== 8 passed in 0.05s ========================

$ pytest tests/test_init.py tests/test_view.py tests/test_cache.py
======================== 47 passed, 1 skipped in 42.38s ========================

The skipped test is the redis-server-dependent test_cache_unlink.

Edge cases

#	Scenario	Input	Expected	Verified by
1	Standard construction	CachedResponse(make_response("hi"), 10)	.timeout == 10, .get_data(as_text=True) == "hi"	test_cached_response_construction_with_flask_response
2	timeout=None passed	CachedResponse(resp, None)	accepted by int | None annotation	annotation allows None
3	inspect.signature introspection	inspect.signature(CachedResponse.__init__)	return annotation is None, params have annotations	test_cached_response_init_is_typed
4	Use in @cached flow	view returns CachedResponse	TTL still respected end-to-end	existing test_cache_timeout_dynamic (unchanged)

Risk / blast radius

Annotation-only change to the public class constructor signature. No
runtime behavior changes: __init__ still copies __dict__ from the
wrapped Response and assigns timeout. The class attribute type
narrows from implicit None to int | None, which matches existing
usage at tests/test_view.py::test_cache_timeout_dynamic and the
isinstance(rv, CachedResponse); rv.timeout access path at
src/flask_caching/__init__.py:432.

---

PR drafted with assistance from Claude Code (Anthropic). The change
was reviewed manually against pallets-eco/flask-caching's source. The
reproducer block above is the one I used during development; reviewers
can paste it verbatim.