fix(guardrails): prevent duplicate prompt-injection-guard log output (#11936)

Boarded with 8 other PRs in one combined worktree: typecheck:core, check:file-size, check:changelog-integrity, check:complexity, check:cognitive-complexity, check:cycles, check-native-deps all green; 75/75 focused tests pass. Verified the console fallback → null fix removes the duplicate plain-text line while the structured pino log is unaffected. Thanks.

src/lib/guardrails/promptInjection.ts

@@ -111,7 +111,7 @@ function shouldBlock(detections: Detection[], threshold: "low" | "medium" | "hig
 }
 
 function getLogger(options: PromptInjectionGuardrailOptions, context: GuardrailContext) {
-  return options.logger || context.log || console;
+  return options.logger ?? context.log ?? null;
 }
 
 function emitGuardrailLog(

src/middleware/promptInjectionGuard.ts

@@ -33,7 +33,7 @@ export function createInjectionGuard(options: PromptInjectionGuardrailOptions =
 
     const decision = evaluatePromptInjection(body, options, {
       disabledGuardrails: resolveDisabledGuardrails({ body }),
-      log: options.logger || console,
+      log: options.logger ?? null,
     });
     return {
       blocked: decision.blocked,