fix(dispatch): forward session_id into registry.dispatch (#28479)

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.

model_tools.py

@@ -1115,6 +1115,7 @@ def handle_function_call(
                     return registry.dispatch(
                         function_name, next_args,
                         task_id=task_id,
+                        session_id=session_id,
                         enabled_tools=sandbox_enabled,
                     )
             else:
@@ -1122,6 +1123,7 @@ def handle_function_call(
                     return registry.dispatch(
                         function_name, next_args,
                         task_id=task_id,
+                        session_id=session_id,
                         user_task=user_task,
                     )
             from hermes_cli.middleware import run_tool_execution_middleware