fix(auth): redact secrets in Debug output for StoredCredentials and StoredAuthorizationState (#744)

* fix(auth): redact secrets in Debug output for StoredCredentials and StoredAuthorizationState

Removes `Debug` from the derive macros on `StoredCredentials` and
`StoredAuthorizationState` and replaces them with manual `Debug` impls
that print `[REDACTED]` for sensitive fields (access/refresh tokens,
PKCE verifiers, and CSRF tokens), preventing accidental credential
leakage via `{:?}` formatters, log calls, and error chains.

Fixes #741

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* test(auth): assert Debug output redacts secrets for credential types

Adds regression tests for the fix in the previous commit, verifying
that `{:?}` formatting of `StoredAuthorizationState` and
`StoredCredentials` does not emit plaintext secrets.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* test(auth): address review feedback on debug redaction tests

- Remove redundant VendorExtraTokenFields from use super:: in
  test_stored_credentials_debug_redacts_token_response (already
  imported at module scope)
- Add assert!(debug_output.contains("created_at")) to
  test_stored_authorization_state_debug_redacts_secrets to verify
  non-secret fields remain visible in Debug output
- Run cargo fmt

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Update crates/rmcp/src/transport/auth.rs

Co-authored-by: Dale Seo <5466341+DaleSeo@users.noreply.github.com>

* fix: remaining formatting issue

* fix: formatting

* fix: formatting

* fix: please

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: Dale Seo <5466341+DaleSeo@users.noreply.github.com>

crates/rmcp/src/transport/auth.rs

@@ -59,7 +59,7 @@ impl<'c> AsyncHttpClient<'c> for OAuthReqwestClient {
 const DEFAULT_EXCHANGE_URL: &str = "http://localhost";
 
 /// Stored credentials for OAuth2 authorization
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Clone, Serialize, Deserialize)]
 pub struct StoredCredentials {
     pub client_id: String,
     pub token_response: Option<OAuthTokenResponse>,
@@ -69,6 +69,20 @@ pub struct StoredCredentials {
     pub token_received_at: Option<u64>,
 }
 
+impl std::fmt::Debug for StoredCredentials {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("StoredCredentials")
+            .field("client_id", &self.client_id)
+            .field(
+                "token_response",
+                &self.token_response.as_ref().map(|_| "[REDACTED]"),
+            )
+            .field("granted_scopes", &self.granted_scopes)
+            .field("token_received_at", &self.token_received_at)
+            .finish()
+    }
+}
+
 /// Trait for storing and retrieving OAuth2 credentials
 ///
 /// Implementations of this trait can provide custom storage backends
@@ -119,13 +133,23 @@ impl CredentialStore for InMemoryCredentialStore {
 }
 
 /// Stored authorization state for OAuth2 PKCE flow
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Clone, Serialize, Deserialize)]
 pub struct StoredAuthorizationState {
     pub pkce_verifier: String,
     pub csrf_token: String,
     pub created_at: u64,
 }
 
+impl std::fmt::Debug for StoredAuthorizationState {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("StoredAuthorizationState")
+            .field("pkce_verifier", &"[REDACTED]")
+            .field("csrf_token", &"[REDACTED]")
+            .field("created_at", &self.created_at)
+            .finish()
+    }
+}
+
 /// A transparent wrapper around a JSON object that captures any extra fields returned by the
 /// authorization server during token exchange that are not part of the standard OAuth 2.0 token
 /// response.
@@ -2776,6 +2800,44 @@ mod tests {
         assert_eq!(deserialized.csrf_token, "my-csrf");
     }
 
+    #[test]
+    fn test_stored_authorization_state_debug_redacts_secrets() {
+        let pkce = PkceCodeVerifier::new("super-secret-verifier".to_string());
+        let csrf = CsrfToken::new("super-secret-csrf".to_string());
+        let state = StoredAuthorizationState::new(&pkce, &csrf);
+        let debug_output = format!("{:?}", state);
+
+        assert!(!debug_output.contains("super-secret-verifier"));
+        assert!(!debug_output.contains("super-secret-csrf"));
+        assert!(debug_output.contains("[REDACTED]"));
+        assert!(debug_output.contains("created_at"));
+        assert!(debug_output.contains("created_at"));
+    }
+
+    #[test]
+    fn test_stored_credentials_debug_redacts_token_response() {
+        use oauth2::{AccessToken, basic::BasicTokenType};
+
+        use super::{OAuthTokenResponse, StoredCredentials};
+
+        let token_response = OAuthTokenResponse::new(
+            AccessToken::new("super-secret-access-token".to_string()),
+            BasicTokenType::Bearer,
+            VendorExtraTokenFields::default(),
+        );
+        let creds = StoredCredentials {
+            client_id: "my-client".to_string(),
+            token_response: Some(token_response),
+            granted_scopes: vec![],
+            token_received_at: None,
+        };
+        let debug_output = format!("{:?}", creds);
+
+        assert!(!debug_output.contains("super-secret-access-token"));
+        assert!(debug_output.contains("[REDACTED]"));
+        assert!(debug_output.contains("my-client"));
+    }
+
     #[test]
     fn test_stored_authorization_state_into_pkce_verifier() {
         let pkce = PkceCodeVerifier::new("original-verifier".to_string());