Expose the PowerShell version in environment context (#41232)

## What changed

- Add the under-development `powershell_shell_version` feature flag.
- When enabled for a single local PowerShell environment, query the selected
  shell executable and include its major/minor version in
  `<environment_context>`.
- Cache version lookups, bound command execution and output, and report when a
  previously visible version becomes unavailable.

## Testing

- Cover environment-context diffs when the shell version appears or disappears.
- Verify on Windows that the version is model-visible only when the feature is
  enabled.

GitOrigin-RevId: 3ec8e80425ec3c193134dc1b69f61ef09ede375e

codex-rs/core/config.schema.json

@@ -817,6 +817,9 @@
             "plugins": {
               "type": "boolean"
             },
+            "powershell_shell_version": {
+              "type": "boolean"
+            },
             "prevent_idle_sleep": {
               "type": "boolean"
             },
@@ -5970,6 +5973,9 @@
         "plugins": {
           "type": "boolean"
         },
+        "powershell_shell_version": {
+          "type": "boolean"
+        },
         "prevent_idle_sleep": {
           "type": "boolean"
         },