fix: pass valid empty MCP config to Claude CLI

[snvtac]

Problem

Fixes #459.

The local Claude Code CLI provider passed --mcp-config "{}" when local CLI config isolation was enabled. Claude Code CLI rejects that shape because it expects an explicit top-level mcpServers record, even when there are no MCP servers.

Root cause

build_claude_cli_args serialized an empty isolated MCP config as {}. Newer Claude Code CLI versions validate that config as missing mcpServers, so provider connection checks can fail before the prompt is sent.

Fix

• Pass {"mcpServers":{}} for the isolated empty MCP config.
• Keep --strict-mcp-config and the existing isolation flags intact.
• Update the Rust unit test to assert both the exact argument value and valid JSON shape.

Validation

• npm run typecheck
• npm run test:mocks -- src/lib/connection-tests.test.ts src/lib/__tests__/claude-cli-transport.test.ts (37 tests)
• npm run build
• Not run locally: cargo test --manifest-path src-tauri/Cargo.toml claude_args_can_isolate_user_config_tools_and_mcp (cargo/rustup are not installed in this shell)

[BookWormSC]

还没修复吗