fix(safety): remove unused import and f-string without placeholders

flake8 fixes:
- _policy.py: remove unused 'os' import (F401)
- _scanner.py: replace f-string with regular string (F541)

Signed-off-by: coder-mtj <coder-mtj@users.noreply.github.com>

Author: coder-mtj

trpc_agent_sdk/tools/safety/_policy.py

@@ -9,7 +9,6 @@
 from __future__ import annotations
 
 import json
-import os
 from pathlib import Path
 
 import yaml

trpc_agent_sdk/tools/safety/_scanner.py

@@ -208,7 +208,7 @@ def _scan_env(req: Request, policy: Policy) -> list[Finding]:
                     DECISION_DENY,
                     RISK_CRITICAL,
                     "sensitive.secret_leak",
-                    [f"environment contains a likely secret"],
+                    ["environment contains a likely secret"],
                     "Remove API keys/tokens from environment variables.",
                 ))
     return findings