fix: clear OAuth error flags when transitioning to StateReady

When a server successfully authenticates via OAuth and transitions to
StateReady, the StateManager was only clearing lastError and retryCount,
but not the OAuth-specific flags (isOAuthError and oauthRetryCount).

This incomplete cleanup could cause:
- Stale error messages in the UI even after successful connection
- Incorrect retry logic treating successful connections as still in error
- Tool count issues if tool indexing depends on clean connection state

This fix ensures all error-related state is reset when a connection
becomes ready, providing a clean slate for the successful connection.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: Dumbris

internal/upstream/types/types.go

@@ -138,6 +138,8 @@ func (sm *StateManager) TransitionTo(newState ConnectionState) {
 	if newState == StateReady {
 		sm.lastError = nil
 		sm.retryCount = 0
+		sm.isOAuthError = false
+		sm.oauthRetryCount = 0
 	}
 
 	info := ConnectionInfo{