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/document_tools.py

@@ -1,8 +1,8 @@
 from typing import Any, Dict, List, Optional
 import logging
 import re
-import json
 from src.constants import MAX_READ_CHARS
+from src.tool_utils import _parse_tool_args
 
 logger = logging.getLogger(__name__)
 
@@ -154,38 +154,6 @@ def _coerce_email_document_content(existing: str, incoming: str) -> str:
         body = new
     return header.rstrip() + "\n---\n" + body
 
-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 — they
-    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
-
 def parse_edit_blocks(content: str) -> list:
     """Parse <<<FIND>>>...<<<REPLACE>>>...<<<END>>> blocks."""
     edits = []