test(exa_search): add unit tests for the ExaSearchChannel doctor branches

[nyxst4ck]

What

Dedicated offline tests for the exa_search channel — the channel had no dedicated test file. 6 tests covering:

• can_handle: always False (search-only channel), pinned so a future URL route can't slip in silently
• check() all five branches:
  • mcporter missing → off with both install prescriptions (npm install -g mcporter + mcporter config add exa)
  • unreadable mcporter config → error (and stale active_backend cleared)
  • exa present in config → warn, never ok — Doctor never probes the remote MCP, so config presence alone must not be sold as usable
  • editor imports left unexpanded → warn (credential-read boundary)
  • mcporter installed but Exa unconfigured → off with the exact add command

How

inspect_mcporter_config and shutil.which are stubbed at the channel module, so every branch runs offline and deterministic — same approach as the merged channel-test series.

Test plan

• pytest tests/test_exa_search_channel.py -v → 6 passed
• Full suite: 416 passed (the 10 failures in test_private_file_writes.py are pre-existing Windows symlink-permission failures on my machine, untouched by this PR)

Follow-up to #331 — continues the dedicated channel coverage series after rss (#360), github (#361), web (#363), reddit (#364), xueqiu (#365), v2ex (#366) and youtube (#367).

[Panniantong]

感谢补充 ExaSearchChannel Doctor 分支测试。merged #577 已在 tests/test_channels.py 覆盖 mcporter 缺失、配置错误、精确 server name、editor imports 与未配置路径，并同步了当前 Exa 工具契约，因此关闭此重复测试 PR。