fix: LSP discovery finds rust-analyzer via rustup proxy

rust-analyzer installed through rustup exits non-zero on --version
("Unknown binary in official toolchain"), which caused discovery
to skip it. Changed command_exists_on_path to treat any successful
spawn as "found", regardless of exit code — only a failure to
spawn (command not found) means the server isn't available.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

rust/crates/runtime/src/lsp_discovery.rs

@@ -100,13 +100,16 @@ pub fn known_lsp_servers() -> Vec<LspServerDescriptor> {
 /// Check whether a command exists on the user's PATH by attempting to run it
 /// with `--version`. Returns `true` if the command could be spawned
 /// successfully, `false` otherwise.
+///
+/// Some LSP servers (like rust-analyzer via rustup) exit non-zero on --version
+/// but are still functional. We treat "spawned successfully" as found, regardless
+/// of the exit code. Only a failure to spawn (command not found) returns false.
 #[must_use]
 pub fn command_exists_on_path(command: &str) -> bool {
     Command::new(command)
         .arg("--version")
         .output()
-        .map(|output| output.status.success())
-        .unwrap_or(false)
+        .map_or(false, |_| true)
 }
 
 /// Discover LSP servers that are actually installed on the current system.