feat: Add support for multiple --mcp-servers-files arguments

[szmania]

Summary

This PR implements support for multiple `--mcp-servers-file` arguments in the CLI, allowing users to specify multiple MCP server configuration files.

Changes

Core Functionality

• Modified argument parsing to accept multiple `--mcp-servers-file` arguments
• Updated MCP server loading logic to merge configurations from multiple files
• Deprecated the old single `--mcp-servers-file` argument in favor of the new multiple-file approach

Files Modified

• `cecli/args.py`: Updated argument parser to support multiple `--mcp-servers-file` arguments
• `cecli/mcp/utils.py`: Enhanced server loading to handle and merge multiple configuration files
• `cecli/main.py`: Updated to pass multiple server file arguments correctly
• `tests/basic/test_main.py`: Fixed test to mock the correct method (`McpServer.connect` instead of `McpServer.start`)

Testing

• All existing tests pass
• New functionality tested with multiple server configuration files

Related Issues

• Addresses CLI-3: Support multiple `--mcp-servers-file` arguments"

#473