Centralize execution setting and session validation

docs/internal/llm.md

@@ -29,7 +29,7 @@ Construction validates the configured base URL and clones any supplied
 `http.Client` so Promptkit can apply its timeout default without mutating the
 caller's client. Generation then:
 
-1. validates request-level timeout and endpoint requirements;
+1. validates shared execution-setting invariants and endpoint requirements;
 2. maps the internal request into the OpenAI-compatible chat payload;
 3. validates and merges extra parameters;
 4. resolves authentication;