settings_content: Make context server args optional (#59623)

# Objective

- Fixes #59614.
- A stdio `context_servers` entry defined with only a `command` (the
minimal form most MCP hosts accept, e.g. `{ "command": "echo" }`) fails
to deserialize. `ContextServerCommand.args` is a required field, so the
entry matches no variant of the untagged `ContextServerSettingsContent`;
the server silently never loads, and the only log line is the opaque
`data did not match any variant of untagged enum
ContextServerSettingsContent`. Other fields (`env`, `timeout`) are
already optional, and the docs only ever show `args` with a value
without stating it is required, so users reasonably assume it is
optional.

## Solution

- Add `#[serde(default)]` to `ContextServerCommand.args` in
`crates/settings_content/src/project.rs`, so it defaults to an empty
list when omitted, the same way `env` and `timeout` are already
optional. `{ "command": "echo" }` now defines a valid stdio server with
no arguments.

## Testing

- Added `test_stdio_context_server_without_args` in
`crates/settings_content/src/project.rs`, which deserializes `{
"command": "echo" }` (asserts empty `args`) and `{ "command": "echo",
"args": ["hello"] }` (regression guard).
- Fail-before/pass-after confirmed: without the change the new test
fails with the exact error from the issue; with it, `cargo test -p
settings_content` passes (24 passed, 0 failed).
- `cargo clippy -p settings_content --tests` and `cargo fmt -p
settings_content -- --check` are clean.
- serde-only change; platform-independent.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

---

Release Notes:

- Fixed stdio MCP servers configured with only a `command` (no `args`)
failing to load.

crates/settings_content/src/project.rs

@@ -481,6 +481,7 @@ pub struct OAuthClientSettings {
 pub struct ContextServerCommand {
     #[serde(rename = "command")]
     pub path: PathBuf,
+    #[serde(default)]
     pub args: Vec<String>,
     pub env: Option<HashMap<String, String>>,
     /// Timeout for tool calls in seconds. Defaults to 60 if not specified.
@@ -847,3 +848,27 @@ pub enum GitHostingProviderKind {
     Forgejo,
     SourceHut,
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_stdio_context_server_without_args() {
+        let settings: ContextServerSettingsContent =
+            serde_json::from_str(r#"{ "command": "echo" }"#)
+                .expect("stdio context server without `args` should parse");
+        let ContextServerSettingsContent::Stdio { command, .. } = settings else {
+            panic!("expected Stdio variant, got {settings:?}");
+        };
+        assert_eq!(command.path, PathBuf::from("echo"));
+        assert!(command.args.is_empty());
+
+        let settings: ContextServerSettingsContent =
+            serde_json::from_str(r#"{ "command": "echo", "args": ["hello"] }"#).unwrap();
+        let ContextServerSettingsContent::Stdio { command, .. } = settings else {
+            panic!("expected Stdio variant, got {settings:?}");
+        };
+        assert_eq!(command.args, vec!["hello".to_string()]);
+    }
+}