feat: add multi-level initial access token support for OAuth 2.0 Dynamic Client Registration (RFC 7591)

[andormarkus]

Add Initial Access Token Support for OAuth 2.0 Dynamic Client Registration (RFC 7591)

Summary

Implements initial access token support for OAuth 2.0 Dynamic Client Registration as specified in RFC 7591. This enables clients to register with protected OAuth endpoints that require initial access tokens.

Changes

Core Implementation

• Add initial_access_token parameter to OAuthClientProvider constructor
• Add initial_access_token() method for custom token retrieval logic
• Implement multi-level fallback system for token resolution
• Add Authorization Bearer header to registration requests when token available
• Add OAUTH_INITIAL_ACCESS_TOKEN environment variable support

Fallback Priority Order

1. Explicit initial_access_token parameter (highest priority)
2. Provider's initial_access_token() method
3. OAUTH_INITIAL_ACCESS_TOKEN environment variable
4. No token (existing behavior)

Testing & Documentation

• Comprehensive test coverage for all fallback scenarios
• Updated README.md with usage examples and Initial Access Tokens section
• Updated simple-auth-client example documentation
• Full backward compatibility maintained

Usage

# Explicit token
oauth_auth = OAuthClientProvider(
    server_url="https://api.example.com",
    client_metadata=client_metadata,
    storage=storage,
    redirect_handler=redirect_handler,
    callback_handler=callback_handler,
    initial_access_token="your-token"
)

# Environment variable
# Set OAUTH_INITIAL_ACCESS_TOKEN=your-token

# Custom provider method
class CustomOAuthProvider(OAuthClientProvider):
    async def initial_access_token(self) -> str | None:
        return await get_token_from_secure_store()

Testing

• ✅ All existing tests pass
• ✅ New initial access token tests: 6/6 passing
• ✅ Auth module tests: 44 passed, 1 xfailed
• ✅ No regressions

Breaking Changes

None. Fully backward compatible.

Other

MCP TypeScript SDK PR - modelcontextprotocol/typescript-sdk#773