[flynt] Fix static-join-to-f-string when mixing raw and non-raw strings (FLY002)

[danparizher]

Summary

Fix FLY002 to bail on transformations when mixing raw and non-raw strings. Mixing them can cause syntax errors, null bytes, or incorrect carriage-return behavior.

Closes #21082

Problem Analysis

When the first string in a join is raw (r"") and later strings are not:

1. The code applies the raw flag to the entire result
2. Non-raw string escape sequences are not processed
3. This leads to:
   • Syntax errors (e.g., "".join((r"", '"')) → r""")
   • Null bytes (e.g., "".join((r"", "\0")))
   • Behavior changes (e.g., carriage returns or unexpected characters)

Approach

Added a check in build_fstring to verify all strings have the same raw status. If not, the transformation is skipped:

1. Collect raw status for all string literals in the join
2. Return None early if raw statuses differ
3. Otherwise, proceed with the transformation

Testing

Added tests covering:

• nok14-nok18: Mixed raw/non-raw (not transformed)
• ok7, ok8: All raw (transformed)
• ok9, ok10: All non-raw (transformed)

Ran with problematic examples from the issue; transformations are skipped as expected.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[dscorbett]

Joining raw and non-raw strings is not the problem. The fix for FLY002 works fine for some combinations:

$ cat >fly002.py <<'# EOF'
print("".join(('"', r"@")))
print("".join((r"@", "!")))
# EOF

$ python fly002.py
"@
@!

$ ruff check --isolated fly002.py --select FLY002 --unsafe-fixes --fix
Found 2 errors (2 fixed, 0 remaining).

$ cat fly002.py
print('"')
print(r"!")

$ python fly002.py
"@
@!

Removing support for such combinations would need to be done in preview, but it would be more helpful to instead keep the support and fix the bugs. In particular, that means that if the fix’s output string cannot be represented as a raw string, it should be represented as a non-raw string, even if the first input string is raw.

[dscorbett]

"\\r" does not have a carriage return escape: it has a backslash escape followed by a literal ⟨r⟩. FLY002 doesn’t change its behavior.