feat(lsp): complete closing triple-quoted strings

[mangeshraut712]

Summary

Fixes #4346

When the cursor is immediately after one, two, or three " / ' quotes, offer a completion that turns them into a closed triple-quoted string (the ty behavior).

• 1–2 quotes: replace the run with """…""" (snippet $0 between the pair when the client supports snippets).
• 3 quotes: insert the closer after the cursor ($0""" with snippets).
• Skip if the next characters are already a matching """ / '''.
• Skip quotes attached to an identifier (foo"), but still fire after string prefixes (f""", r""", …).

AI usage: an assistant drafted the completion helper and tests; I reviewed the LSP edit ranges, prefix handling, and test cursors.

Test Plan

cargo test -p pyrefly --lib -- completion_closes_triple -- completion_expands_partial -- completion_skips_triple -- completion_skips_quotes -- completion_closes_prefixed

All five new tests passed.

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