fix(weixin): convert MSYS / Git-Bash drive paths to native Windows on send

[xxxigm]

Summary

Closes #31457.

On Windows, when the agent runs under Git Bash / MSYS the gateway
sees MEDIA paths like /c/Users/.../cache/file.jpg (and the matching
file:///c/Users/... URL). Native Windows Python's pathlib.Path
does not understand that drive shape — it joins the leading / to
the current drive's root, so the canonical example resolves to a
non-existent C:\c\Users\... and Path(path).read_bytes() reports
[Errno 2] No such file or directory. The Weixin adapter therefore
silently drops every native attachment, even though Telegram and
Discord deliver the exact same path fine because they upload bytes
through a different code path that doesn't hit the MSYS quirk.

Approach

Two small, well-tested helpers in gateway/platforms/base.py:

• normalize_msys_path(path) — Windows-only rewrite of
  /c/Users/... and the legacy /cygdrive/c/... shape to
  C:/Users/..., drive letter ASCII-uppercased, no-op on POSIX so a
  real /c filesystem path is never corrupted.
• strip_file_url_prefix(url) — strip exactly the file://
  scheme delimiter, drop the synthetic third slash before a
  <drive>: segment so callers get C:/foo instead of /C:/foo,
  and route the remainder through normalize_msys_path so the
  file:///c/Users/... shape (lowercase drive, no colon) survives.

Wired in at the validation seam (validate_media_delivery_path),
the bare-path scanner (extract_local_files) and the Weixin
adapter's two file-touching call sites (send_image's old naive
replace("file://", "") and _send_file's Path(path).read_bytes()).

Test plan

• pytest tests/gateway/test_msys_path_normalization_31457.py —
  25 new tests covering both helpers, source-level guards on
  the four wired call sites, and a behavioural smoke test that
  the existing happy-path validation still works.
• pytest tests/gateway/test_platform_base.py tests/gateway/test_media_extraction.py tests/gateway/test_weixin.py — 153 passed, 2 skipped (no
  regressions in the surrounding adapters).
• Manual: agent emits MEDIA:/c/Users/.../cache/x.jpg under
  Git Bash on Windows 11 → image is delivered natively to WeChat
  (matching Telegram/Discord behaviour).

Made with Cursor

[teknium1]

Thanks for tracing this Windows-specific delivery path. The underlying defect is still present on current main: gateway/platforms/base.py:1295 constructs Path from the unconverted candidate, while gateway/platforms/weixin.py:1991 strips file:// naively before _send_file reaches Path(path).read_bytes().

Problems

• The new converter duplicates the existing Windows-gated conversion in tools/environments/local.py:23-48, which already handles /c, /cygdrive, and /mnt forms. The PR copy omits /mnt, creating a second, narrower contract.
• tests/gateway/test_msys_path_normalization_31457.py asserts source text via inspect.getsource() rather than delivery behavior. These assertions would fail after a valid refactor and do not prove the filesystem/send boundary receives a native path.

Suggested changes

• Consolidate on one low-dependency MSYS normalization utility and use it at the current validation and Weixin file-read seams.
• Replace source-inspection assertions with behavioral tests for conversion before validation/existence checks and for Weixin forwarding file:///c/... to its local-send path.

Automated hermes-sweeper review.