* Fix ASYNC200 regression on `foo("x").bar` patterns (#26.4.2)

The canonical-qualname resolver introduced in 26.4.1 recursed through
ast.Call/cst.Call unconditionally, which collapsed nested calls inside an
attribute chain into a dotted name (e.g. `read_session("a").get` resolved
to `read_session.get`). This caused user-configured ASYNC200 patterns
like `*session.get` to flag method calls on the return value of an
unrelated function -- a false positive that did not exist in 25.5.3.

Only unwrap Call at the outermost position; calls inside an attribute
chain now make the whole expression unresolvable (None), since the
target of the trailing attribute is a return value we can't determine
statically. Top-level shapes like `trio.open_nursery()` still resolve
as before.

Adds:
  - eval-file regression case in tests/eval_files/async200.py
  - direct unit test for resolve_canonical_ast / resolve_canonical_cst
  - changelog entry for 26.4.2 and version bump

https://claude.ai/code/session_01BWheK2fZPMDhbfS1LCR2zc

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Satisfy pre-commit: hoist import + black reformat

- Move `resolve_canonical_ast` / `resolve_canonical_cst` import to the top
  of tests/test_flake8_async.py (ruff PLC0415 / import-not-at-top).
- Black reformat of `_resolve_attr_chain_ast` signature.

https://claude.ai/code/session_01BWheK2fZPMDhbfS1LCR2zc

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>