fix: request identity encoding for streamable HTTP

[he-yufeng]

Summary

• send Accept-Encoding: identity on streamable HTTP client requests
• keep the MCP Accept and Content-Type request headers overriding client defaults
• add coverage that the actual server-visible request headers use identity encoding

Fixes #2649.

To verify

• uv run pytest tests/shared/test_streamable_http.py::test_streamable_http_client_mcp_headers_override_defaults tests/shared/test_streamable_http.py::test_streamable_http_client_preserves_custom_with_mcp_headers -q --basetemp .tmp/pytest
• uv run pytest tests/shared/test_streamable_http.py::test_streamable_http_client_json_response -q --basetemp .tmp/pytest-json
• uv run ruff check src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py
• uv run python -m py_compile src/mcp/client/streamable_http.py tests/shared/test_streamable_http.py
• git diff --check