feat: add HTTP client option to OAuthConfig (#616)

* feat: add HTTP client option to OAuthConfig

It's already possible to customize the underlying client used by the SSE
and streamable HTTP transports, but not the OAuth handler. This adds
an option to customize the underlying client used by the OAuth handler.

* Use conventional comment on HTTPClient

Author: sd2k

client/transport/oauth.go

@@ -34,6 +34,9 @@ type OAuthConfig struct {
 	AuthServerMetadataURL string
 	// PKCEEnabled enables PKCE for the OAuth flow (recommended for public clients)
 	PKCEEnabled bool
+	// HTTPClient is an optional HTTP client to use for requests.
+	// If nil, a default HTTP client with a 30 second timeout will be used.
+	HTTPClient *http.Client
 }
 
 // TokenStore is an interface for storing and retrieving OAuth tokens.
@@ -151,10 +154,13 @@ func NewOAuthHandler(config OAuthConfig) *OAuthHandler {
 	if config.TokenStore == nil {
 		config.TokenStore = NewMemoryTokenStore()
 	}
+	if config.HTTPClient == nil {
+		config.HTTPClient = &http.Client{Timeout: 30 * time.Second}
+	}
 
 	return &OAuthHandler{
 		config:     config,
-		httpClient: &http.Client{Timeout: 30 * time.Second},
+		httpClient: config.HTTPClient,
 	}
 }