Fixes #795.

Summary

This updates the SSE client endpoint resolution logic for API-gateway and nested-route deployments.

Previously, sse_client used urljoin(url, sse.data) for endpoint events. If the public SSE URL included a gateway prefix and the server emitted an absolute endpoint path, Python treated the endpoint as origin-rooted and dropped the gateway prefix.

For example:

https://example.com/gateway/deployment/v1/sse + /v1/messages/?session_id=...

previously resolved to:

https://example.com/v1/messages/?session_id=...

The new resolver preserves the non-overlapping public prefix when the SSE URL path and endpoint path overlap:

https://example.com/gateway/deployment/v1/messages/?session_id=...

It intentionally keeps standard root-relative behavior for no-overlap absolute paths such as /messages, because those are ambiguous: the client cannot safely infer whether the endpoint should be rooted at the origin or under a gateway prefix. Server-side mount-prefix cases should continue to be handled through ASGI root_path.

Verification

• uv run --frozen pytest tests/shared/test_sse.py -q
• uv run --frozen ruff format --check src/mcp/client/sse.py tests/shared/test_sse.py
• uv run --frozen ruff check src/mcp/client/sse.py tests/shared/test_sse.py
• uv run --frozen pyright src/mcp/client/sse.py tests/shared/test_sse.py
• targeted coverage for src/mcp/client/sse.py: 100%
• UV_FROZEN=1 uv run --frozen strict-no-cover
• ./scripts/test