Fix suppressions after nested multiline f-strings

[loulanyue]

Summary

• detect suppression-like text hidden by the lightweight multiline-string scanner
• use Ruff's existing parser tokens only for that rare ambiguous case
• preserve the lightweight fast path for normal modules
• cover Pyrefly/Pyre suppressions and ensure string contents remain ignored

The lightweight scanner loses the surrounding single-quoted f-string context across physical lines. For a nested multiline string, its closing triple quotes can therefore be mistaken for a new top-level string opener, hiding a valid suppression on a later line. The scoped token fallback uses Python's actual lexical structure without parsing every module twice.

Fixes #4118.

Verification

• cargo test -p pyrefly_python: 71 passed, 0 failed
• cargo test -p pyrefly_python ignore: 8 passed, 0 failed
• cargo clippy -p pyrefly_python --all-targets -- -D warnings: passed
• python3 test.py --no-test --no-tensor-shapes --no-conformance --no-jsonschema: passed; existing unrelated warnings remain on Rust 1.97.1
• cargo run -p pyrefly -- check /tmp/pyrefly-4118-repro.py: 0 errors (1 suppressed)
• git diff --check: passed

AI disclosure

This contribution was prepared and reviewed with Codex acting as an AI coding agent. The issue reproduction, fallback scope, negative test, end-to-end behavior, formatting, tests, and lint results were explicitly verified before submission.

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