Disable env-bound tools when exec server is none (#16349)

## Summary
- make `CODEX_EXEC_SERVER_URL=none` map to an explicit disabled
environment mode instead of inferring from a missing URL
- expose environment capabilities (`exec_enabled`, `filesystem_enabled`)
so tool building can gate behavior explicitly and future
multi-environment work has a clearer seam
- suppress env-backed tools when the relevant capability is unavailable,
including exec tools, `js_repl`, `apply_patch`, `list_dir`, and
`view_image`
- keep handler/runtime backstops so disabled environments still reject
execution if a tool path somehow bypasses registration

## Testing
- `just fmt`
- `cargo test -p codex-exec-server`
- `cargo test -p codex-tools
disabled_environment_omits_environment_backed_tools`
- `cargo test -p codex-tools
environment_capabilities_gate_exec_and_filesystem_tools_independently`
- remote devbox Bazel build via `codex-applied-devbox`:
`//codex-rs/cli:cli`

codex-rs/core/src/unified_exec/mod_tests.rs

@@ -98,7 +98,7 @@ async fn exec_command_with_tty(
                 &request,
                 tty,
                 Box::new(NoopSpawnLifecycle),
-                turn.environment.as_ref(),
+                turn.environment.as_ref().expect("turn environment"),
             )
             .await?,
     );
@@ -593,7 +593,7 @@ async fn remote_exec_server_rejects_inherited_fd_launches() -> anyhow::Result<()
 
     let remote_test_env = remote_test_env().await?;
     let (_, mut turn) = make_session_and_context().await;
-    turn.environment = Arc::new(remote_test_env.environment().clone());
+    turn.environment = Some(Arc::new(remote_test_env.environment().clone()));
 
     let request = test_exec_request(
         &turn,
@@ -611,7 +611,7 @@ async fn remote_exec_server_rejects_inherited_fd_launches() -> anyhow::Result<()
             Box::new(TestSpawnLifecycle {
                 inherited_fds: vec![42],
             }),
-            turn.environment.as_ref(),
+            turn.environment.as_ref().expect("turn environment"),
         )
         .await
         .expect_err("expected inherited fd rejection");

codex-rs/core/src/unified_exec/process_manager.rs

@@ -593,7 +593,7 @@ impl UnifiedExecProcessManager {
             .ok_or(UnifiedExecError::MissingCommandLine)?;
         let inherited_fds = spawn_lifecycle.inherited_fds();
 
-        if environment.exec_server_url().is_some() {
+        if environment.is_remote() {
             if !inherited_fds.is_empty() {
                 return Err(UnifiedExecError::create_process(
                     "remote exec-server does not support inherited file descriptors".to_string(),