fix(channels): make contract checks portable and complete

[kaiwangleo]

Closes #7264

What Problem This Solves

The existing channel contract gate can report success without proving that every registered built-in channel has a runnable contract test. On a Windows checkout using a non-UTF-8 default code page, the previous checker read UTF-8 Python sources with the platform default encoding and could raise UnicodeDecodeError. Its filename/regular-expression scan also missed SIPChannel, which is exported from sip/__init__.py, and did not reliably cover OneBot, WeChat, WeCom, XiaoYi, or Yuanbao.

The local channel runner had corresponding portability and discovery gaps: it used legacy package checks and extras, could misclassify changed files, and did not consistently include untracked files or preserve required failure status. This made a green local result insufficient evidence of complete built-in channel coverage.

Evidence

The baseline behavior was reproduced by inspecting the checker and registry: the checker used implicit Path.read_text() encoding, scanned only channel.py, and reported incomplete coverage for the registered channels. The runner also contained the legacy package name and derived changed channels from filenames rather than registry keys.

The implementation in this PR is verified by these concrete results:

• python -X utf8=0 scripts/check_channel_contracts.py reports 18 total, 18 tested, and 0 missing channels.
• WSL Python 3.12 checker, runner, and channel-contract regression tests: 448 passed, 1 skipped.
• pytest tests/contract -q: 406 passed, 1 skipped.
• pytest tests/unit -q: 8065 passed, 19 skipped.
• The SIP runner executed 19 contract tests and 5 supplemental unit tests successfully.
• bash -n scripts/check-channels.sh, repository pre-commit hooks, Mypy, Black, Flake8, Pylint, actionlint, and targeted git diff --check passed.

Summary

• Replace filename and regular-expression discovery with an explicit UTF-8 AST checker driven by the built-in registry.
• Recognize package-level exports such as SIPChannel, validate canonical BaseChannel inheritance and contract paths, and fail closed on ambiguous or disabled contract factories.
• Add isolated contract coverage for OneBot, SIP, WeChat, WeCom, XiaoYi, and Yuanbao, and remove the Console meta-test that could pass by skipping.
• Harden scripts/check-channels.sh for QwenPaw package/test extras, linked worktrees, tracked and untracked changes, registry-based mapping, and required-check exit codes.
• Add checker/runner regression tests, LF normalization, documentation, and CI coverage checks. The workflow's code-change filter includes the checker, runner, and .gitattributes paths.

Scope and compatibility

This PR covers built-in channel contract coverage and its local/CI gates. Plugin-provided channels remain responsible for coverage in their own test scope. Optional channel unit tests remain supplemental and non-blocking in the local runner.

The implementation is organized into three focused commits so the coverage fixtures, static gate, and local runner can be reviewed independently. The branch is synchronized with the current upstream main; maintainers can split the commits into separate PRs if preferred.

No optional channel dependency is imported by the static checker.

[github-actions]

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Hi @kaiwangleo, thank you for your first Pull Request! 🎉

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