Add turn-scoped environment selections (#18416)

## Summary
- add experimental turn/start.environments params for per-turn
environment id + cwd selections
- pass selections through core protocol ops and resolve them with
EnvironmentManager before TurnContext creation
- treat omitted selections as default behavior, empty selections as no
environment, and non-empty selections as first environment/cwd as the
turn primary

## Testing
- ran `just fmt`
- ran `just write-app-server-schema`
- not run: unit tests for this stacked PR

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/rmcp-client/src/program_resolver.rs

@@ -70,7 +70,7 @@ mod tests {
     #[tokio::test]
     async fn test_unix_executes_script_without_extension() -> Result<()> {
         let env = TestExecutableEnv::new()?;
-        let mut cmd = Command::new(&env.program_name);
+        let mut cmd = Command::new(&env.executable_path);
         cmd.envs(&env.mcp_env);
 
         let output = cmd.output().await;
@@ -138,6 +138,8 @@ mod tests {
         // Held to prevent the temporary directory from being deleted.
         _temp_dir: TempDir,
         program_name: String,
+        #[cfg(unix)]
+        executable_path: std::path::PathBuf,
         mcp_env: HashMap<OsString, OsString>,
     }
 
@@ -160,6 +162,8 @@ mod tests {
             let mcp_env = create_env_for_mcp_server(Some(extra_env), &[])?;
 
             Ok(Self {
+                #[cfg(unix)]
+                executable_path: Self::executable_path(dir_path),
                 _temp_dir: temp_dir,
                 program_name: Self::TEST_PROGRAM.to_string(),
                 mcp_env,
@@ -184,6 +188,11 @@ mod tests {
             Ok(())
         }
 
+        #[cfg(unix)]
+        fn executable_path(dir: &Path) -> std::path::PathBuf {
+            dir.join(Self::TEST_PROGRAM)
+        }
+
         #[cfg(unix)]
         fn set_executable(path: &Path) -> Result<()> {
             use std::os::unix::fs::PermissionsExt;