fix: show loading state when switching adaptor docs

[elias-ba]

Description

This PR fixes the docs panel showing stale content when switching a job's adaptor. The useDocs hook didn't reset its state to null (loading) when fetching a new uncached adaptor, so the previous adaptor's docs stayed visible until the new ones loaded.

Closes #4461

Validation steps

1. Open a workflow and select a job
2. Open the docs panel
3. Change the job's adaptor to a different one
4. Verify "Loading docs..." appears while the new docs are fetched

Additional notes for the reviewer

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• I have not used AI

Pre-submission checklist

• I have performed an AI review of my code (we recommend using /review with Claude Code)
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[josephjclark]

Great!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.42%. Comparing base (dbd5f3f) to head (46d2395).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4462      +/-   ##
==========================================
- Coverage   89.47%   89.42%   -0.06%     
==========================================
  Files         425      425              
  Lines       20213    20213              
==========================================
- Hits        18086    18075      -11     
- Misses       2127     2138      +11     

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