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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Pre-submission checklist

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[josephjclark]

Great!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.44%. Comparing base (ec09f30) to head (0280ad8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4462      +/-   ##
==========================================
- Coverage   89.48%   89.44%   -0.04%     
==========================================
  Files         425      425              
  Lines       20303    20303              
==========================================
- Hits        18169    18161       -8     
- Misses       2134     2142       +8     

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