Summary

Fixes #6144. Adding an LLM provider (the reporter used "claude") and clicking Test left the Save button greyed out, so the provider appeared impossible to add even though the connection test passed.

Root cause

The configure drawer's testKey (in useProviderConfig) does two things on a Test click:

1. Persists the entered key via set_provider_key (the backend test endpoint validates the stored credential, so it has to be saved first), then sets keyInput: "" so the password field shows its "existing key" placeholder.
2. Runs the test mutation and records testResult.

Step 1's clearing of keyInput makes isUnchanged evaluate to true (empty key input, unchanged base/proxy URL), and isUnchanged fed directly into saveDisabled. So after a passing Test the Save button greyed out — even though the credential was already persisted by the Test itself. To the user that reads as "this provider can't be added".

Fix

Keep Save enabled when the form is otherwise unchanged but a Test has just passed (testResult.ok). The credential is already saved at that point, so clicking Save runs the no-op save path and dismisses the drawer with the usual success toast — a clear confirm-and-close affordance instead of a dead-looking button.

-  const saveDisabled = !config.provider || config.saving || config.testing || isUnchanged;
+  const testedOk = config.testResult?.ok === true;
+  const saveDisabled = !config.provider || config.saving || config.testing || (isUnchanged && !testedOk);

Verification

• Added a regression test in ProvidersPage.test.tsx: open the Add picker, pick an unconfigured provider, enter a key, click Test, and assert Save stays enabled. Confirmed it fails on the pre-fix saveDisabled condition and passes after the fix.
• pnpm test --run — 786 passed (80 files)
• pnpm typecheck — clean
• pnpm lint — 0 errors (pre-existing baseline warnings only, none in the touched files)
• pnpm build — ok