debugger: Mark DebugAdapterBinary::program as optional (#32534)

This allows us to support debugging with a debug adapter not managed by
Zed. Note that this is not a user facing change, as DebugAdapterBinary
is used to determine how to spawn a debugger. Thus, this should not
break any configs or anything like that.

Closes #ISSUE

Release Notes:

- N/A

crates/dap_adapters/src/python.rs

@@ -187,7 +187,7 @@ impl PythonDebugAdapter {
         );
 
         Ok(DebugAdapterBinary {
-            command: python_command,
+            command: Some(python_command),
             arguments,
             connection: Some(adapters::TcpArguments {
                 host,