Both the regular and execute_code dispatch paths forward task_id into
registry.dispatch via middleware _dispatch lambdas but silently dropped
session_id. Dispatch-layer hooks (e.g. set_enforcement_fn) that correlate
calls with the active session received "" for every invocation.
Pass session_id=session_id at both _dispatch call sites inside
handle_function_call, matching the existing task_id pattern. Hooks
already received session_id; this closes the registry.dispatch gap.
Rebased onto current main where dispatch is wrapped by
run_tool_execution_middleware — the old direct-dispatch sites from
#28479 no longer exist.
test(dispatch): add tests for session_id forwarding (NousResearch#28479)
Covers standard and execute_code paths through the middleware wrapper.
Verifies task_id forwarding is not broken by the change.
