fix(graph): bound concurrency caps by the semaphore permit limit

Semaphore::new panics above tokio's MAX_PERMITS, so a fixed or templated
map cap (or settings.max_concurrency) that large crashed the run instead
of erroring. The validator rejects both at load, resolve_max_concurrency
rejects every arm at run time, and the executor checks the graph setting
before building its own semaphore, so validation mode makes no difference.

Also corrects run_child_agent_with_graph_inputs' contract comment: inputs
for a config-only agent are rejected, not ignored.

src/function/agents.rs

@@ -565,7 +565,8 @@ impl Drop for CancelOnDrop {
 
 /// Runs a child agent to completion. When the child is a graph agent,
 /// `graph_inputs` is overlaid on its `initial_state` before the graph
-/// starts; it is ignored for config-only agents.
+/// starts. Passing inputs to a config-only agent is rejected before the
+/// child runs, since nothing could consume them.
 fn run_child_agent_with_graph_inputs(
     mut child_ctx: RequestContext,
     initial_input: Input,