fix(auth): per-project credential activation (#51152) - #52674

Issue for this PR

Closes #51152

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

• Allows project/folder configuration (providers.<id>.auth) to pin which stored credential (by ID or label) is active for sessions under that directory, with the global active account serving as fallback.
• Adds per-location credential pinning to Integration.Service (connection.pin, unpin), ensuring GET /api/integration/{id} connection ordering and active connection resolution vary by project location rather than leaking machine-wide.
• Extends Credential.Service to store multiple credentials per integration with explicit activation (activate) and promotion upon deletion, maintaining global active credential fallback.
• Documents precedence rules: pinned credential > global active credential > environment connection, with stored/env connection taking precedence over explicit settings.apiKey.
• Adds regression unit tests in packages/core covering multi-credential storage, activation, promotion, location pinning, and config integration.

How did you verify your code works?

• Tested with bun test test/config/provider.test.ts test/credential.test.ts test/integration.test.ts in packages/core. All 15 tests pass cleanly.

Screenshots / recordings

N/A

Checklist

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• I have added tests to cover my changes.
• All new and existing tests passed.