fix(oauth): remove credential caching to enable automatic token refresh

The DatabaseCredentialStore was caching StoredCredentials with stale expires_in values, preventing RMCP's automatic token refresh from working correctly. After idle periods, tool calls would fail with 401 errors even though tokens should have been refreshed.

Changes:
- Removed cache field and all caching logic from DatabaseCredentialStore
- expires_in is now recalculated on every load() from the stored expires_at timestamp
- RMCP's AuthClient can now properly detect expired tokens and refresh them automatically
- Added comprehensive tests to verify token expiration detection and refresh behavior

Fixes: OAuth-protected servers (like Atlassian MCP) failing after idle periods

Author: Mohammod Al Amin Ashik