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/tools/src/tool_config.rs

@@ -86,6 +86,7 @@ pub struct ToolsConfig {
     pub shell_type: ConfigShellToolType,
     pub shell_command_backend: ShellCommandBackendConfig,
     pub unified_exec_shell_mode: UnifiedExecShellMode,
+    pub has_environment: bool,
     pub allow_login_shell: bool,
     pub apply_patch_tool_type: Option<ApplyPatchToolType>,
     pub web_search_mode: Option<WebSearchMode>,
@@ -200,6 +201,7 @@ impl ToolsConfig {
             shell_type,
             shell_command_backend,
             unified_exec_shell_mode: UnifiedExecShellMode::Direct,
+            has_environment: true,
             allow_login_shell: true,
             apply_patch_tool_type,
             web_search_mode: *web_search_mode,
@@ -236,6 +238,11 @@ impl ToolsConfig {
         self
     }
 
+    pub fn with_has_environment(mut self, has_environment: bool) -> Self {
+        self.has_environment = has_environment;
+        self
+    }
+
     pub fn with_unified_exec_shell_mode(
         mut self,
         unified_exec_shell_mode: UnifiedExecShellMode,