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/tool_utils.py

@@ -4,6 +4,8 @@ src.constants which imports nothing from src). Adding a project import here
 will reintroduce the circular dependency that this module exists to break.
 """
 
+import json
+
 from src.constants import MAX_OUTPUT_CHARS
 
 _mcp_manager = None
@@ -37,3 +39,36 @@ def _truncate(text: str, limit: int = MAX_OUTPUT_CHARS) -> str:
     if len(text) > limit:
         return text[:limit] + f"\n... (truncated, {len(text)} chars total)"
     return text
+
+
+def _parse_tool_args(content):
+    """Parse a tool-call argument blob.
+
+    Accepts either a JSON string or an already-decoded dict. Unwraps the
+    common `{"body": {...}}` envelope that smaller models emit when they
+    read tool descriptions like "Body is JSON: {...}" literally and
+    pass `body` as a field name rather than treating it as a noun.
+
+    Returns a dict on success, raises ValueError on bad JSON.
+    """
+    if isinstance(content, str):
+        try:
+            args = json.loads(content) if content.strip() else {}
+        except (json.JSONDecodeError, TypeError) as e:
+            raise ValueError(str(e))
+    elif isinstance(content, dict):
+        args = content
+    else:
+        args = {}
+    # Unwrap {"body": {...}} envelope, but only if `body` is the sole key
+    # and points at a dict. We don't want to clobber a legitimate `body`
+    # field on tools where it's a real arg (e.g. send_email body text).
+    if (
+        isinstance(args, dict)
+        and len(args) == 1
+        and "body" in args
+        and isinstance(args["body"], dict)
+        and "action" in args["body"]  # extra safety: only unwrap if the inner dict looks like a tool call
+    ):
+        args = args["body"]
+    return args