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/tools/handlers/view_image.rs

@@ -91,9 +91,13 @@ impl ToolHandler for ViewImageHandler {
             AbsolutePathBuf::try_from(turn.resolve_path(Some(args.path))).map_err(|error| {
                 FunctionCallError::RespondToModel(format!("unable to resolve image path: {error}"))
             })?;
+        let Some(environment) = turn.environment.as_ref() else {
+            return Err(FunctionCallError::RespondToModel(
+                "view_image is unavailable in this session".to_string(),
+            ));
+        };
 
-        let metadata = turn
-            .environment
+        let metadata = environment
             .get_filesystem()
             .get_metadata(&abs_path)
             .await
@@ -110,8 +114,7 @@ impl ToolHandler for ViewImageHandler {
                 abs_path.display()
             )));
         }
-        let file_bytes = turn
-            .environment
+        let file_bytes = environment
             .get_filesystem()
             .read_file(&abs_path)
             .await

codex-rs/core/src/tools/runtimes/unified_exec.rs

@@ -239,7 +239,12 @@ impl<'a> ToolRuntime<UnifiedExecRequest, UnifiedExecProcess> for UnifiedExecRunt
             .await?
             {
                 Some(prepared) => {
-                    if ctx.turn.environment.exec_server_url().is_some() {
+                    let Some(environment) = ctx.turn.environment.as_ref() else {
+                        return Err(ToolError::Rejected(
+                            "exec_command is unavailable in this session".to_string(),
+                        ));
+                    };
+                    if environment.is_remote() {
                         return Err(ToolError::Rejected(
                             "unified_exec zsh-fork is not supported when exec_server_url is configured".to_string(),
                         ));
@@ -251,7 +256,7 @@ impl<'a> ToolRuntime<UnifiedExecRequest, UnifiedExecProcess> for UnifiedExecRunt
                             &prepared.exec_request,
                             req.tty,
                             prepared.spawn_lifecycle,
-                            ctx.turn.environment.as_ref(),
+                            environment.as_ref(),
                         )
                         .await
                         .map_err(|err| match err {
@@ -281,13 +286,18 @@ impl<'a> ToolRuntime<UnifiedExecRequest, UnifiedExecProcess> for UnifiedExecRunt
         let exec_env = attempt
             .env_for(command, options, req.network.as_ref())
             .map_err(|err| ToolError::Codex(err.into()))?;
+        let Some(environment) = ctx.turn.environment.as_ref() else {
+            return Err(ToolError::Rejected(
+                "exec_command is unavailable in this session".to_string(),
+            ));
+        };
         self.manager
             .open_session_with_exec_env(
                 req.process_id,
                 &exec_env,
                 req.tty,
                 Box::new(NoopSpawnLifecycle),
-                ctx.turn.environment.as_ref(),
+                environment.as_ref(),
             )
             .await
             .map_err(|err| match err {