fix: allow credentials on common adaptor

[elias-ba]

Description

This PR fixes credential support for the @openfn/language-common adaptor. Previously, the UI showed "This adaptor does not require credentials" and hid all credential options. We just found out that sometimes users need raw JSON credentials on common adaptor steps.

Validation steps

1. Open a workflow and select a job using @openfn/language-common
2. Click "Connect" to open the configure modal
3. Verify that existing credentials (raw JSON, HTTP, OAuth, keychain) are listed
4. Verify that adaptor-specific credentials (e.g., Salesforce) are not shown in the main list
5. Click "New Credential" and verify the Raw JSON credential form opens without errors
6. Save the credential and confirm it gets attached to the job
7. Repeat steps 2-3 with a non-common adaptor (e.g., Salesforce) and confirm credential filtering still works as before

Additional notes for the reviewer

The edit credential flow is unaffected — when editing, the backend loads the existing credential which already has a valid schema stored.

AI Usage

• I have used Claude Code
• I have used another model
• I have not used AI

Pre-submission checklist

• I have performed an AI review of my code (we recommend using /review with Claude Code)
• I have implemented and tested all related authorization policies. (e.g., OpenFn, :admin, :editor, :viewer)
• I have updated the changelog.
• I have ticked a box in "AI usage" in this PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.54%. Comparing base (5d4762c) to head (b6a5d2f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4513      +/-   ##
==========================================
- Coverage   89.55%   89.54%   -0.01%     
==========================================
  Files         425      425              
  Lines       20307    20307              
==========================================
- Hits        18185    18184       -1     
- Misses       2122     2123       +1     

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[josephjclark]

Thanks @elias-ba !

[midigofrank]

@elias-ba I have converted this to draft. Waiting for product decision

[elias-ba]

Thank you @midigofrank that's the right thing to do.