fix(agents): make tool call ID sanitization conditional with standard/strict modes

- Add ToolCallIdMode type ('standard' | 'strict') for provider compatibility
- Standard mode (default): allows [a-zA-Z0-9_-] for readable session logs
- Strict mode: only [a-zA-Z0-9] for Mistral via OpenRouter
- Update sanitizeSessionMessagesImages to accept toolCallIdMode option
- Export ToolCallIdMode from pi-embedded-helpers barrel

Addresses review feedback on PR #1372 about readability.

src/agents/pi-embedded-helpers.ts

@@ -50,4 +50,5 @@ export {
 } from "./pi-embedded-helpers/turns.js";
 export type { EmbeddedContextFile, FailoverReason } from "./pi-embedded-helpers/types.js";
 
+export type { ToolCallIdMode } from "./tool-call-id.js";
 export { isValidCloudCodeAssistToolId, sanitizeToolCallId } from "./tool-call-id.js";