feat: allow overriding SSE messages endpoint

[HenryLee789]

Summary

• add an optional messages_url override to sse_client() for reverse-proxy deployments
• preserve session query parameters from the server endpoint event when an override is used
• cover absolute and relative overrides plus end-to-end client POST routing

Fixes #2255

Checks

• uv run pytest tests/shared/test_sse.py -q
• uv run ruff format --check src/mcp/client/sse.py tests/shared/test_sse.py
• uv run ruff check src/mcp/client/sse.py tests/shared/test_sse.py
• uv run pyright src/mcp/client/sse.py tests/shared/test_sse.py
• uv run coverage combine && uv run coverage report --include='src/mcp/client/sse.py' (100%)
• git diff --check