Add unit tests for OAuth.refreshAccessToken()

[Copilot]

The refresh-token exchange logic was moved into OAuth.refreshAccessToken() without corresponding unit tests. This adds focused coverage for that method.

Tests added (OAuth.test.ts)

• Success path — token store is updated with new access + refresh tokens; new access token is returned
• Request shape — verifies grant_type, refresh_token, and client_id are sent correctly
• Failure paths:
  • Non-OK HTTP response → null, token store unchanged
  • Invalid JSON body → null
  • Missing access_token or refresh_token in response → null
  • Missing tokenStore or basePath config → null
  • No refresh token in store → null
  • fetch throws (network error) → null

jsdom is not installed in this package, so the browser-only constructor guard is satisfied by stubbing window via vi.stubGlobal.

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