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

[peatey]

Summary

Fixes #741.

StoredCredentials and StoredAuthorizationState both derived #[derive(Debug)], causing secret fields — OAuth access/refresh tokens, PKCE verifiers, and CSRF tokens — to be printed in plaintext via any {:?} formatter, including log calls and anyhow/thiserror error chains.

• Removes Debug from the derive macros on both types
• Implements Debug manually, printing [REDACTED] for sensitive fields and leaving non-secret fields (e.g. client_id, created_at) visible
• Adds regression tests asserting that {:?} formatting does not emit plaintext secrets

Approach notes

OAuthTokenResponse comes from the oauth2 crate and cannot be wrapped directly, so StoredCredentials uses a manual Debug impl that redacts the entire token_response field.

The secrecy crate was considered for StoredAuthorizationState (wrapping pkce_verifier/csrf_token in SecretString), but secrecy intentionally blocks Serialize on Secret<T> unless the inner type implements the SerializableSecret marker trait — which String does not, and orphan rules prevent adding it. Since both types are Serialize + Deserialize for use with custom CredentialStore/StateStore backends, field-type changes would break serialization. Manual Debug impls are the correct approach for both types.

Known scope limitation

The fields pkce_verifier: String and csrf_token: String on StoredAuthorizationState remain pub. This fix protects against accidentally formatting the struct via {:?}, but not against a caller extracting the field and formatting it directly (e.g. format!("{:?}", state.pkce_verifier)). This is an inherent limitation of the manual-Debug approach vs. type-level wrapping, and is noted here for reviewer awareness.

Test plan

• cargo test -p rmcp --features auth passes
• cargo clippy -p rmcp --features auth clean (pre-existing unused import warning unrelated to this change)
• test_stored_authorization_state_debug_redacts_secrets — asserts {:?} output does not contain raw verifier/csrf values and does contain [REDACTED]
• test_stored_credentials_debug_redacts_token_response — asserts {:?} output does not contain raw access token and does contain [REDACTED]

[DaleSeo]

Thanks for your contribution, @peatey! Just a couple of small suggestions below. Also, please ensure the code is formatted to pass the check.

[peatey]

ok fmt now run (its in the workflow so not sure what happened there, sorry) - issues addressed.

[DaleSeo]

Hey @jamadeo, could you help approve/merge this PR? I've fixed the formatting issue on the web, but now it seems like someone else needs to approve it since I've added commits technically. 😂