feat(python): expose OAuth connection config (#3586)

Expose the merged Rust OAuth header provider through the Python async
connection path.

Includes:
- Python OAuthConfig and OAuthFlowType public config objects
- PyO3 conversion into the Rust OAuthConfig
- connect_async(oauth_config=...) plumbing
- repr redaction coverage for client_secret

Local validation: cargo fmt --all; ruff format/check on touched Python
files.

python/src/lib.rs

@@ -26,6 +26,7 @@ pub mod expr;
 pub mod header;
 pub mod index;
 pub mod namespace;
+pub mod oauth;
 pub mod permutation;
 pub mod query;
 pub mod runtime;