feat: support principal propagation on list mcp tools

[NicoleMGomes]

Disclaimer: Do not include SAP-internal or customer-specific information in this PR (e.g. internal system URLs, customer names, tenant IDs, or confidential configurations). This is a public repository.

Description

Adds an optional user_token parameter to AgentGatewayClient.list_mcp_tools(). When provided, the method uses user-scoped authentication (Phase 2 / token exchange) instead of the default system-scoped authentication, enabling principal propagation during tool discovery — consistent with how call_mcp_tool() already works.

• LoB agents: if user_token is supplied, calls fetch_user_auth() (token exchange via IAS user fragment) instead of fetch_system_auth().
• Customer agents: if user_token is supplied, calls exchange_user_token() (jwt-bearer grant via mTLS) instead of get_system_token_mtls().

Both flows are backwards-compatible: omitting user_token preserves the existing system-token behaviour.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactoring
• Dependency update

How to Test

1. Create an AgentGatewayClient for a LoB agent with a valid tenant_subdomain.
2. Call await agw_client.list_mcp_tools(user_token="<valid-user-jwt>") and verify tools are returned with user-scoped auth (check token exchange happens instead of client credentials).
3. Call await agw_client.list_mcp_tools() (no user_token) and verify the existing system-token behaviour is unchanged.
4. Repeat steps 2–3 for a Customer agent (file-based credentials).
5. Run the unit tests: uv run pytest tests/agentgateway/unit/test_agw_client.py::TestListMcpTools -v

Checklist

• I have read the Contributing Guidelines
• I have verified that my changes solve the issue
• I have added/updated automated tests to cover my changes
• All tests pass locally
• I have verified that my code follows the Code Guidelines
• I have updated documentation (if applicable)
• I have added type hints for all public APIs
• My code does not contain sensitive information (credentials, tokens, etc.)
• I have followed Conventional Commits for commit messages

Additional Notes

The user_token parameter accepts both a plain str and a Callable[[], str] (consistent with call_mcp_tool and get_user_auth), allowing lazy token resolution at call time.