refactor(tools): migrate config/integration admin tools to the registry (#4742)

Part of #3629 (the `admin_tools.py` bullet). Moves the config/integration admin
tools off the legacy elif dispatch chain in tool_implementations.py onto the
agent_tools registry:

  manage_endpoints, manage_mcp, manage_webhooks, manage_tokens, manage_settings

The do_* implementations (and manage_mcp's command-allowlist / RCE guard:
_validate_mcp_command, _mcp_allowed_commands, and the _MCP_* constants) move
verbatim into the new src/agent_tools/admin_tools.py. They register through a
single ADMIN_TOOL_HANDLERS map that TOOL_HANDLERS.update()s, and the five elif
branches plus their imports are dropped from tool_execution.py, so these tools
now flow through _direct_fallback like the other migrated clusters. The names
are re-exported from src.agent_tools for back-compat.

Dedup:
  - _parse_tool_args was duplicated in tool_implementations.py and
    document_tools.py. It now lives once in src.tool_utils (which imports nothing
    from the project beyond src.constants, so this introduces no cycle) and both
    call sites import it from there. The orphaned `import json` in document_tools
    is removed with it.
  - The five tools share one _owner_adapter(fn) factory that threads ctx["owner"]
    into the owner-taking do_* signature, instead of five near-identical wrappers.

Tests: new tests/test_admin_tools_registry.py pins the registration, the
re-export back-compat, the owner-threading adapter, and the single-source
_parse_tool_args (across admin_tools and document_tools). Existing MCP /
settings / webhook suites are repointed at the new module.

src/agent_tools/__init__.py

@@ -25,6 +25,11 @@ from .document_tools import CreateDocumentTool, UpdateDocumentTool, EditDocument
 from .model_interaction_tools import ChatWithModelTool, AskTeacherTool, ListModelsTool
 from .bg_job_tools import ManageBgJobsTool
 from .session_tools import CreateSessionTool, ListSessionsTool, SendToSessionTool, ManageSessionTool
+from .admin_tools import (
+    ADMIN_TOOL_HANDLERS,
+    do_manage_endpoints, do_manage_mcp, do_manage_webhooks,
+    do_manage_tokens, do_manage_settings,
+)
 
 TOOL_HANDLERS = {
     "bash": BashTool().execute,
@@ -52,6 +57,8 @@ TOOL_HANDLERS = {
     "send_to_session": SendToSessionTool().execute,
     "manage_session": ManageSessionTool().execute,
 }
+# Config/integration admin tools (manage_endpoints/mcp/webhooks/tokens/settings).
+TOOL_HANDLERS.update(ADMIN_TOOL_HANDLERS)
 
 # ---------------------------------------------------------------------------
 # Constants (re-exported for backward compatibility — single source of truth
@@ -138,10 +145,5 @@ from src.tool_implementations import (  # noqa: E402, F401
     do_search_chats,
     do_manage_skills,
     do_manage_tasks,
-    do_manage_endpoints,
-    do_manage_mcp,
-    do_manage_webhooks,
-    do_manage_tokens,
-    do_manage_settings,
     do_api_call,
 )