[ruff] Indented form feeds (RUF054) (#16049)

## Summary

Resolves #12321.

The physical-line-based `RUF054` checks for form feed characters that
are preceded by only tabs and spaces, but not any other characters,
including form feeds.

## Test Plan

`cargo nextest run` and `cargo insta test`.

scripts/check_docs_formatted.py

@@ -97,6 +97,7 @@ KNOWN_FORMATTING_VIOLATIONS = [
 KNOWN_PARSE_ERRORS = [
     "blank-line-with-whitespace",
     "indentation-with-invalid-multiple-comment",
+    "indented-form-feed",
     "missing-newline-at-end-of-file",
     "mixed-spaces-and-tabs",
     "no-indented-block",