fix(mcp): Restore McpTool.__init__ body and fix method name during rebase

- Restore missing __init__ method body that was lost during rebase
- Add proper instance variable initialization (_mcp_tool, _mcp_session_manager,
  _require_confirmation, _header_provider, _progress_callback)
- Rename get_function_declaration to _get_declaration with @OverRide decorator
  to match upstream convention

This fixes test failures in test_mcp_toolset.py where tests expect
_progress_callback attribute on McpTool instances.

Author: timof1308

src/google/adk/tools/mcp_tool/mcp_tool.py

@@ -166,6 +166,27 @@ def __init__(
             The factory receives (tool_name, callback_context, **kwargs) and
             returns a ProgressFnT or None. This allows callbacks to access
             and modify runtime context like session state.
+
+    Raises:
+        ValueError: If mcp_tool or mcp_session_manager is None.
+    """
+    super().__init__(
+        name=mcp_tool.name,
+        description=mcp_tool.description if mcp_tool.description else "",
+        auth_config=AuthConfig(
+            auth_scheme=auth_scheme, raw_auth_credential=auth_credential
+        )
+        if auth_scheme
+        else None,
+    )
+    self._mcp_tool = mcp_tool
+    self._mcp_session_manager = mcp_session_manager
+    self._require_confirmation = require_confirmation
+    self._header_provider = header_provider
+    self._progress_callback = progress_callback
+
+  @override
+  def _get_declaration(self) -> FunctionDeclaration:
     """Gets the function declaration for the tool.
 
     Returns: