MCP: Fix scene_action connect not persisting signal connections

[MichaelFisher1997]

Problem

The scene_action tool's connect action reported success ("Connected signal…") but never persisted the connection: no [connection] entry was written to the .tscn file (verified by grepping both existing and freshly created scenes). The connection existed only in memory for the running session, so AI agents wiring up signals got a misleading success message plus a generated callback stub while the actual wire-up silently vanished on save/reload.

Root cause

MCPTools::tool_scene_action() connected signals with default flags:

source->connect(sig, Callable(target, method));

PackedScene::pack() → SceneState::_parse_connections() skips every connection without CONNECT_PERSIST (scene/resources/packed_scene.cpp:1086), so the packed scene dropped it. The editor's Connections dialog sets this flag (editor/scene/connections_dialog.cpp:950); the MCP tool did not.

Fix

• Pass CONNECT_PERSIST when connecting (modules/mcp/mcp_tools.cpp).
• Check connect()'s return value and surface an error instead of a success message on failure.

Verification

Ran the editor binary headless as an MCP server and drove it over stdio JSON-RPC:

1. initialize handshake
2. scene_action create → fresh res://test.tscn (Node2D root)
3. scene_action add → Button child
4. scene_action connect → pressed → on_pressed

Before: .tscn contained no [connection] section.
After:

[connection signal="pressed" from="MyButton" to="." method="on_pressed"]

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