test(linkedin): add unit tests for the LinkedInChannel doctor branches

[nyxst4ck]

What

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

• can_handle: linkedin.com host match (case-insensitive) and rejection of lookalike hosts
• check() all five branches:
  • mcporter missing → off with the Jina Reader fallback hint
  • unreadable mcporter config → error (and stale active_backend cleared)
  • LinkedIn MCP present in config (all 3 accepted aliases) → warn, never ok — Doctor must not claim usability without a live probe
  • editor imports left unexpanded → warn (credential-read boundary)
  • mcporter installed but LinkedIn unconfigured → off with the exact mcporter config add linkedin prescription

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_linkedin_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 LinkedIn Doctor 分支测试。merged #577 已按当前 mcp-server-linkedin、uvx 和 stdio 契约覆盖缺失、配置错误、未安装 uvx、已配置但未实测及 editor imports 等路径；本 PR 仍断言旧 linkedin-scraper-mcp 提示，因此作为已取代测试关闭。