fix: address Gadfly findings on kimi provider

- kimi:// DSN scheme: missing-credential hint now names the LLM_<NAME> env
  var that defines the provider (its token comes from the DSN, not
  KIMI_API_KEY), matching providerFor's lazy-resolution key form. Fixes the
  correctness/error-handling findings that the old hint misdirected users to
  set KIMI_API_KEY when the fix is adding a token to the DSN.
- parse_test.go: add kimi to TestBuiltinsResolve. (llama-swap stays excluded
  and is now documented — its no-URL built-in errors at Model() construction,
  not just on use, so it can't resolve there; the finding's llama-swap half
  was a false lead the test surfaced.)
- Add TestKimiSchemeMissingToken covering the corrected hint.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

parse_test.go

@@ -213,7 +213,9 @@ func TestBuiltinsResolve(t *testing.T) {
 	r := newTestRegistry(t)
 	// All built-in provider names resolve even before their client
 	// implementations land (stub providers error only on use).
-	for _, name := range []string{"openai", "anthropic", "google", "ollama", "ollama-cloud", "foreman"} {
+	// Note: llama-swap is intentionally excluded — its no-URL built-in errors
+	// at Model() construction (not just on use), so it can't resolve here.
+	for _, name := range []string{"openai", "kimi", "anthropic", "google", "ollama", "ollama-cloud", "foreman"} {
 		if _, err := r.Parse(name + "/anything"); err != nil {
 			t.Errorf("Parse(%s/anything): %v", name, err)
 		}