Add client analytics headers

[ewlarson]

Summary

• Add shared MCP client header helpers for X-BTAA-Client-* attribution and User-Agent values.
• Send MCP client headers from the HTTP bridge and from the WebSocket bridge when the optional ws implementation is available.
• Update the QGIS plugin client channel to qgis and cover the expected analytics headers in tests.
• Document the MCP header environment variables and the current first-party client identities.

Why

The analytics program needs stable first-party client attribution across desktop and MCP clients, not only browser and SSR traffic. These headers let downstream analytics distinguish QGIS, MCP HTTP bridge, and MCP WebSocket bridge requests.

Impact

MCP bridge and QGIS traffic can now be attributed with explicit client name, version, channel, and optional instance metadata. The WebSocket bridge warns when it falls back to a WebSocket implementation that cannot attach custom handshake headers.

Validation

• /Users/ewlarson/.cache/codex-runtimes/codex-primary-runtime/dependencies/node/bin/node --test mcp/client_headers.test.js mcp/mcp_http_bridge.test.js mcp/mcp_websocket_bridge.test.js
• /Users/ewlarson/.pyenv/versions/3.13.1/bin/python -m pytest qgis-plugin/tests

Note: python -m pytest qgis-plugin/tests passed with an existing pytest-asyncio deprecation warning about asyncio_default_fixture_loop_scope.