## Why
Remote HTTP MCP connections can source header values from the host environment,
so those variables must remain part of the connection identity even when the MCP
server runs in an executor environment.
## What changed
- Track `env_http_headers` variables for remote HTTP MCP connections.
- Continue tracking `bearer_token_env_var` only for local environments, where the
host resolves the token.
## Testing
Add coverage that verifies remote connections track host header variables without
tracking executor bearer-token variables, while local connections track both.
GitOrigin-RevId: 8fe175c54a101721a27b76495dcd298f0c2bc6a3