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/src/client.rs

@@ -297,7 +297,7 @@ mod tests {
         let client = DebugAdapterClient::start(
             crate::client::SessionId(1),
             DebugAdapterBinary {
-                command: "command".into(),
+                command: Some("command".into()),
                 arguments: Default::default(),
                 envs: Default::default(),
                 connection: None,
@@ -367,7 +367,7 @@ mod tests {
         let client = DebugAdapterClient::start(
             crate::client::SessionId(1),
             DebugAdapterBinary {
-                command: "command".into(),
+                command: Some("command".into()),
                 arguments: Default::default(),
                 envs: Default::default(),
                 connection: None,
@@ -420,7 +420,7 @@ mod tests {
         let client = DebugAdapterClient::start(
             crate::client::SessionId(1),
             DebugAdapterBinary {
-                command: "command".into(),
+                command: Some("command".into()),
                 arguments: Default::default(),
                 envs: Default::default(),
                 connection: None,