Move azsdk start command to azsdk mcp

[benbp]

In the tools/azsdk-cli app, the current command line argument to run the MCP server is start. This should be renamed to mcp.

To verify, the below command:

dotnet run --project Azure.Sdk.Tools.Cli -- --help

Should show:

... other before help text ...
Commands:
  eng                  Internal azsdk engineering system commands
  log                  Log processing commands
  mcp                  Run in MCP (model context protocol) server mode
... other after help text ...

Instead of:

... other before help text ...
Commands:
  eng                  Internal azsdk engineering system commands
  log                  Log processing commands
  start
... other after help text ...

Note: Do NOT try to run the start/mcp command, as it will start the server process and would need to be canceled with ctrl-c!