chore: prune unused imports and duplicate import redefinitions

Remove unused imports (F401) and duplicate/shadowed import
redefinitions (F811) across the codebase using ruff's safe
autofixes. No behavioral changes -- imports only.

- ~1400 safe autofixes applied across 644 files (net -1072 lines)
- __init__.py re-exports preserved (excluded from F401 removal so
  public re-export surfaces stay intact)
- Re-exports that are imported or monkeypatched by tests but look
  unused in their defining module are kept with explicit # noqa:
  F401 (gateway/run.py load_dotenv; run_agent re-exports from
  agent.message_sanitization, agent.context_compressor,
  agent.retry_utils, agent.prompt_builder, agent.process_bootstrap,
  agent.codex_responses_adapter)
- Unsafe F841 (unused-variable) fixes deliberately skipped -- those
  can change behavior when the RHS has side effects
- ruff lints remain disabled in pyproject.toml (only PLW1514 is
  selected); this is a one-time cleanup, not a config change

Verification:
- python -m compileall: clean
- pytest --collect-only: all 27161 tests collect (zero import errors)
- core entry points import clean (run_agent, model_tools, cli,
  toolsets, hermes_state, batch_runner, gateway)
- static scan: every name any test imports directly from an edited
  module still resolves

agent/chat_completion_helpers.py

@@ -15,49 +15,23 @@ sites unchanged.  Symbols that tests patch on ``run_agent`` (e.g.
 
 from __future__ import annotations
 
-import concurrent.futures
-import contextvars
-import copy
 import json
 import logging
 import os
-import random
 import re
-import sys
 import threading
 import time
 import uuid
-from datetime import datetime
-from pathlib import Path
 from types import SimpleNamespace
-from typing import Any, Dict, List, Optional, Tuple
-from urllib.parse import urlparse, parse_qs, urlunparse
+from typing import Any, Dict, Optional
 
 from hermes_cli.timeouts import get_provider_request_timeout, get_provider_stale_timeout
 from hermes_constants import PARTIAL_STREAM_STUB_ID, FINISH_REASON_LENGTH
-from agent.error_classifier import classify_api_error, FailoverReason
+from agent.error_classifier import FailoverReason
 from agent.model_metadata import is_local_endpoint
 from agent.message_sanitization import (
     _sanitize_surrogates,
-    _sanitize_messages_surrogates,
-    _sanitize_structure_surrogates,
-    _sanitize_messages_non_ascii,
-    _sanitize_tools_non_ascii,
-    _sanitize_structure_non_ascii,
-    _strip_images_from_messages,
-    _strip_non_ascii,
     _repair_tool_call_arguments,
-    _escape_invalid_chars_in_json_strings,
-)
-from agent.tool_dispatch_helpers import (
-    _is_multimodal_tool_result,
-    _multimodal_text_summary,
-)
-from agent.retry_utils import jittered_backoff
-from agent.tool_guardrails import (
-    ToolGuardrailDecision,
-    append_toolguard_guidance,
-    toolguard_synthetic_result,
 )
 from tools.terminal_tool import is_persistent_env
 from utils import base_url_host_matches, base_url_hostname