Support remote MCP servers over SSE/HTTP and harden the MCP client runtime

[shadowinlife]

Summary

This PR extends the built-in agent's MCP client support from stdio-only to remote SSE and streamable HTTP servers.

It also hardens the runtime path by removing retry-on-timeout for remote tool calls, moving MCP registry construction off the async session loop, and making transport-switching config overrides safe.

Why

Before this change, the built-in agent could load external MCP tools only through local stdio subprocesses. Remote HTTP-based MCP servers were not actually supported.

Supporting remote transports also required fixing a few correctness issues around retries, async startup, and config merging.

Testing

• Existing tests pass (pytest --ignore=agent/tests/e2e_backtest --tb=short -q)
• New tests added (if applicable)
• Tested manually (describe below)

Checklist

• No changes to protected areas (src/agent/, src/session/, src/providers/) without prior discussion
• No hardcoded values (API keys, file paths, magic numbers)
• Code follows CONTRIBUTING.md guidelines
• Documentation updated (if user-facing change)

[Copilot]

Pull request overview

This PR extends the agent’s MCP client from stdio-only subprocess servers to also support remote MCP servers over SSE and streamable HTTP, and hardens startup/config behavior to avoid unsafe retries and event-loop blocking during registry construction.

Changes:

• Add SSE and streamable HTTP MCP transports (client adapter + schema validation) and document the new config fields.
• Harden MCP runtime: remove retry-on-timeout for remote tool calls and build the MCP/tool registry off the async session loop.
• Add integration + regression tests covering SSE/HTTP MCP discovery/calls, enabledTools filtering, unreachable-server behavior, and registry-build responsiveness.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
README.md	Documents new MCP transport fields and updated transport support notes.
agent/SKILL.md	Updates MCP server configuration docs for stdio vs HTTP transports.
agent/src/tools/mcp.py	Adds SSE/streamable HTTP transports and removes timeout retries for remote tool calls.
agent/src/config/schema.py	Validates transport-specific MCP server config; requires explicit type for URL-based servers.
agent/src/config/loader.py	Makes override merging transport-aware and safely falls back on invalid merged configs.
agent/src/session/service.py	Moves registry construction into the executor to keep the async loop responsive.
agent/tests/test_agent_config.py	Expands schema + override-merge tests for new transport behaviors.
agent/tests/test_mcp_client_adapter.py	Adds adapter unit tests for transport selection + updated retry behavior.
agent/tests/test_session_service_mcp.py	Regression test ensuring registry build doesn’t block the event loop.
agent/tests/mcp_http_test_helpers.py	Adds shared subprocess/port/ready-check helpers for HTTP MCP integration tests.
agent/tests/test_mcp_sse_integration.py	Full-stack SSE MCP integration tests with a real FastMCP server subprocess.
agent/tests/test_mcp_streamable_http_integration.py	Full-stack streamable HTTP MCP integration tests with a real FastMCP server subprocess.
agent/tests/test_mcp_stdio_integration.py	Marks stdio MCP integration tests as integration.
agent/tests/fixtures/fake_mcp_sse_server.py	Minimal FastMCP SSE server fixture for integration tests.
agent/tests/fixtures/fake_mcp_streamable_http_server.py	Minimal FastMCP streamable HTTP server fixture for integration tests.

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