suppress: avoid orphaning foreign linter pragmas

[nitishagar]

Summary

pyrefly suppress (default LineBefore mode) inserts # pyrefly: ignore [...] on the line above a target. When the line directly above already contains a non-pyrefly linter pragma — # noqa, # nosemgrep, # pyright: ignore, # pylint:, # nosec, # ruff:, # flake8:, foreign-tool # type: ignore — the new pyrefly comment slides between them, breaking the foreign pragma's adjacency to its target line and silently disabling it.

This PR detects that case and appends # pyrefly: ignore [...] to the end of the target line instead, leaving the foreign pragma untouched. Behavior for plain comments and other situations is unchanged.

• Adds has_foreign_linter_pragma(line) helper (uses the existing find_comment_start_in_line so string literals don't false-match).
• Wires it into add_suppressions's LineBefore branch via a force_inline flag.
• 6 new tests covering noqa, nosemgrep, pyright, plain-comment-unchanged, string-not-comment, and the helper itself.

Fixes #3373

Test plan

• cargo test -p pyrefly --lib — 5161/5161 passing locally (includes the 6 new tests).
• Existing test_add_suppressions_existing_comment (where the line above is a plain comment, not a pragma) still passes — confirms behavior is unchanged for non-pragma comments.
• CI green.

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[rchen152]

Hi @nitishagar - thanks for the PR! We can review it (and any other PRs from you) as soon as you sign the CLA (see the instructions here).

[nitishagar]

@rchen152 I've signed the CLA. Let me know if you have any questions or anything I can clarify on the above changes.

[meta-cla]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Meta Open Source project. Thanks!

[meta-cla]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Meta Open Source project. Thanks!

[rchen152]

Thanks! @yangdanny97 would you mind taking a look at this one, since it looks like you had started thinking about #3373?

[nitishagar]

Rebased onto latest main and resolved the conflicts (Cargo.lock + suppress.rs) — ready for review whenever convenient. Thanks!