fix(v2ex): recover from Python TLS EOF

[LeonChaoX]

Summary

• cap V2EX urllib responses at 1 MiB
• retry only Python TLS unexpected-EOF failures through the native curl TLS stack
• preserve fail-closed behavior for certificate verification and ordinary network failures
• add regressions for fallback behavior and doctor recovery

Root cause

On some Windows/network combinations, Python's TLS stack raises UNEXPECTED_EOF_WHILE_READING even though the same V2EX endpoint succeeds through the operating system's curl. Because the channel and doctor shared the same urllib path, Agent Reach reported V2EX as unavailable and could not fetch data.

User impact

V2EX checks and data calls now recover when the native TLS stack can reach the endpoint, without weakening certificate validation or hiding unrelated failures.

Validation

• python -m pytest tests/test_v2ex_channel.py tests/test_doctor.py tests/test_channel_contracts.py tests/test_skill_command.py -q — 37 passed
• python -m ruff check agent_reach/channels/v2ex.py tests/test_v2ex_channel.py tests/test_skill_command.py
• git diff --check

Closes #514

[Panniantong]

感谢实现 V2EX TLS EOF fallback。你的原始作者提交已保留在 merged #577 中，并在其上增加了精确错误边界、响应限制、回归测试和真实 smoke；因此关闭原 PR。