Document request validation workflow

engine.go

@@ -355,13 +355,16 @@ func (e *Engine) Prepare(ctx context.Context, req RunRequest) (*PreparedRun, err
 
 // ValidateRequest reports whether req passes the same preparation path used by
 // [Engine.Prepare] at the time of this call. A nil error means this concrete
-// request passed preparation. The method returns no prepared snapshot or
-// executable handle. It does not call an LLM, but preparation may still perform
-// resource I/O, including loading supplied inputs and schemas.
+// request passed preparation. It checks only this invocation and returns errors
+// as encountered rather than aggregating them. All supplied inputs follow the
+// same loading behavior as [Engine.Prepare], including inputs unused by the
+// selected template branch. The method does not call an LLM or retain a
+// prepared snapshot or executable handle, but preparation may still perform
+// resource I/O and incur its associated time and memory costs.
 //
 // Its errors and context handling follow [Engine.Prepare], including an error
 // matching ErrInvalidConfig when the Engine is nil. Success does not guarantee
-// that a provider will accept a later request. Use [Engine.PrepareExecution]
+// provider acceptance or later execution success. Use [Engine.PrepareExecution]
 // when execution must use the snapshot produced by preparation.
 func (e *Engine) ValidateRequest(ctx context.Context, req RunRequest) error {
 	_, err := e.prepareRequest(ctx, req)