Fix: UpdateMCPProxy collapses transient DB errors into not-found response

Summary

In gateway/gateway-controller/pkg/utils/mcp_deployment.go, the UpdateMCPProxy function currently collapses every error from GetMCPProxyByHandle into a generic "not found" message. This means transient DB/read failures are surfaced as false 404-like errors instead of internal errors.

Fix Required

Differentiate between true not-found errors and transient/unexpected errors in the UpdateMCPProxy lookup:

existing, err := s.GetMCPProxyByHandle(handle)
if err != nil {
    if isMCPNotFoundError(err) {
        return nil, fmt.Errorf("MCP proxy configuration with handle '%s' not found", handle)
    }
    return nil, fmt.Errorf("failed to look up MCP proxy: %w", err)
}
if existing == nil {
    return nil, fmt.Errorf("MCP proxy configuration with handle '%s' not found", handle)
}

Context

• Flagged during code review of PR #1514 (comment: https://github.com/wso2/api-platform/pull/1514#discussion_r3000447264)
• Deferred to a follow-up PR by @dushaniw

/cc @dushaniw