Make environment provider snapshots path-free (#21794)

## Summary
- make EnvironmentProvider::snapshot path-free and keep providers
focused on provider-owned remote environments
- let provider snapshots request local inclusion via include_local, with
environments.toml including local and CODEX_EXEC_SERVER_URL excluding
local
- move reserved local environment construction into EnvironmentManager
using ExecServerRuntimePaths

Follow-up to https://github.com/openai/codex/pull/20667

## Testing
- just fmt
- git diff --check
- devbox: bazel build --bes_backend= --bes_results_url=
//codex-rs/exec-server:exec-server
- devbox: bazel test --bes_backend= --bes_results_url=
//codex-rs/exec-server:exec-server-unit-tests

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

codex-rs/app-server/tests/suite/v2/turn_start.rs

@@ -1991,7 +1991,7 @@ async fn turn_start_updates_sandbox_and_cwd_between_turns_v2() -> Result<()> {
 }
 
 #[tokio::test]
-async fn turn_start_resolves_sticky_thread_environments_and_turn_overrides() -> Result<()> {
+async fn turn_start_resolves_sticky_thread_local_environment_and_turn_overrides() -> Result<()> {
     let tmp = TempDir::new()?;
     let codex_home = tmp.path().join("codex_home");
     std::fs::create_dir(&codex_home)?;
@@ -2000,12 +2000,16 @@ async fn turn_start_resolves_sticky_thread_environments_and_turn_overrides() ->
 
     let server = create_mock_responses_server_repeating_assistant("done").await;
     create_config_toml(&codex_home, &server.uri(), "never", &BTreeMap::default())?;
+    std::fs::write(
+        codex_home.join("environments.toml"),
+        r#"
+[[environments]]
+id = "remote"
+url = "ws://127.0.0.1:1"
+"#,
+    )?;
 
-    let mut mcp = McpProcess::new_with_env(
-        &codex_home,
-        &[("CODEX_EXEC_SERVER_URL", Some("http://127.0.0.1:1"))],
-    )
-    .await?;
+    let mut mcp = McpProcess::new(&codex_home).await?;
     timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;
 
     for case in [
@@ -2024,16 +2028,6 @@ async fn turn_start_resolves_sticky_thread_environments_and_turn_overrides() ->
             sticky: Some(&["local"]),
             turn: None,
         },
-        EnvironmentSelectionCase {
-            name: "sticky_remote_turn_unset",
-            sticky: Some(&["remote"]),
-            turn: None,
-        },
-        EnvironmentSelectionCase {
-            name: "sticky_local_remote_turn_unset",
-            sticky: Some(&["local", "remote"]),
-            turn: None,
-        },
         EnvironmentSelectionCase {
             name: "sticky_local_turn_empty",
             sticky: Some(&["local"]),
@@ -2044,21 +2038,6 @@ async fn turn_start_resolves_sticky_thread_environments_and_turn_overrides() ->
             sticky: Some(&[]),
             turn: Some(&["local"]),
         },
-        EnvironmentSelectionCase {
-            name: "sticky_local_turn_remote",
-            sticky: Some(&["local"]),
-            turn: Some(&["remote"]),
-        },
-        EnvironmentSelectionCase {
-            name: "sticky_remote_turn_local",
-            sticky: Some(&["remote"]),
-            turn: Some(&["local"]),
-        },
-        EnvironmentSelectionCase {
-            name: "sticky_unset_turn_local_remote",
-            sticky: None,
-            turn: Some(&["local", "remote"]),
-        },
     ] {
         run_environment_selection_case(&mut mcp, &workspace, case).await?;
     }