refactor(tools): extract cookbook domain into src/tools/cookbook.py

Moves the model-serving (cookbook) tool domain out of tool_implementations.py
into src/tools/cookbook.py as part of slice 1 (#4082/#4071):

- 13 do_* tools: download/serve/list/stop/tail/search/adopt/cached models,
  list downloads/cancel, list cookbook servers, serve presets
- 9 private helpers: _cookbook_servers, _resolve_cookbook_host,
  _cookbook_env_for_host, _infer_serve_{port,host}, _ensure_served_endpoint,
  _cookbook_register_task, _cookbook_apply_retry_suggestion,
  _scan_running_model_processes, _cookbook_kill_session
- _MODEL_PROCESS_PATTERNS constant (used only by _scan_running_model_processes)

tool_implementations.py stays a backward-compatible facade via a re-import
from src.tools.cookbook; src/tools/__init__ re-exports the same symbols.

_internal_headers and _INTERNAL_BASE stay in tool_implementations.py (shared
by system.py's do_app_api and many cookbook funcs). Each cookbook function
that needs them does a function-local import to avoid a top-level circular
dependency, matching the system-domain split.

Verified: compileall clean; shim test green; cookbook-touching suite
(652 passed, 1 skipped); full suite 3587 passed, 2 failed
(pre-existing test_api_chat_security, unrelated).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/tools/__init__.py

@@ -9,3 +9,15 @@ from src.tools.system import (  # noqa: F401
     do_manage_mcp, do_manage_webhooks, do_manage_tokens, do_manage_settings,
     do_api_call, do_app_api,
 )
+from src.tools.cookbook import (  # noqa: F401
+    do_download_model, do_serve_model, do_list_served_models,
+    do_stop_served_model, do_tail_serve_output, do_list_downloads,
+    do_cancel_download, do_search_hf_models, do_adopt_served_model,
+    do_list_cookbook_servers, do_list_serve_presets, do_serve_preset,
+    do_list_cached_models,
+    _cookbook_servers, _resolve_cookbook_host, _cookbook_env_for_host,
+    _infer_serve_port, _infer_serve_host, _ensure_served_endpoint,
+    _cookbook_register_task, _cookbook_apply_retry_suggestion,
+    _scan_running_model_processes, _cookbook_kill_session,
+    _MODEL_PROCESS_PATTERNS,
+)