framework tools/git: pr-review validates --action before provider contact (codex blocker on PR #1464)

An unsupported --action previously reached platform detection (and
could touch the provider) before failing with a provider-class status;
comment-without-body exited 1 mid-switch. Both now fail fast with
usage_error (rc 2, stderr) immediately after argument parsing. Test
arms added including the zero-provider-contact assertion for
invalid-action runs.

packages/mosaic/framework/tools/git/pr-review.sh

@@ -116,6 +116,14 @@ if [[ -z "$ACTION" ]]; then
     usage_error "Action is required (-a/--action): approve, request-changes, comment"
 fi
 
+# Validate the action BEFORE any provider contact (codex review of PR #1464:
+# an unsupported --action previously reached platform detection and could
+# touch the provider before failing with a provider-class status).
+case "$ACTION" in
+    approve|request-changes|comment) ;;
+    *) usage_error "unknown action '$ACTION': approve, request-changes, comment" ;;
+esac
+
 if [[ -n "$REPO_OVERRIDE" ]]; then
     # An explicit --repo is the whole point of a reviewer worktree whose origin
     # is nonstandard or missing (#867 convention, mirrored from pr-view.sh /
@@ -715,8 +723,7 @@ if [[ "$PLATFORM" == "github" ]]; then
             echo "Added review comment to GitHub PR #$PR_NUMBER"
             ;;
         *)
-            echo "Error: Unknown action: $ACTION"
-            exit 1
+            usage_error "unknown action: $ACTION"
             ;;
     esac
 elif [[ "$PLATFORM" == "gitea" ]]; then
@@ -809,8 +816,7 @@ elif [[ "$PLATFORM" == "gitea" ]]; then
             echo "Added and verified comment on Gitea PR #$PR_NUMBER (comment ID $comment_id)"
             ;;
         *)
-            echo "Error: Unknown action: $ACTION"
-            exit 1
+            usage_error "unknown action: $ACTION"
             ;;
     esac
 else