test(auth): add SetTestUserDID helper for integration tests
Add SetTestUserDID() function to inject user DID into context for testing.
Purpose:
- Mock authenticated users in integration tests without full OAuth flow
- Used by post handler tests to simulate authenticated requests
- Marked with comment: "ONLY be used in tests"
This enables testing authenticated endpoints (like post creation)
without requiring real PDS authentication in test environment.
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Co-Authored-By: Claude <noreply@anthropic.com>