[mangeshraut712]

Summary

Fixes #4346.

When the cursor is immediately after exactly three matching quotes (""" or '''), offer a triple-quoted string completion that inserts the matching closer after the cursor. When the client supports snippets, the cursor placeholder is placed before the closer.

• Supports valid Python string prefixes: r, u, f, b, fr, rf, br, and rb.
• Does not offer a completion after one or two quotes, which already open ordinary string literals.
• Does not offer a completion when matching closing triple quotes already follow, after a non-prefix identifier, or inside an existing string or comment.

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_

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[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D119010817. (Because this pull request was imported automatically, there will not be any future comments.)

[mangeshraut712]

CLA is signed and the diff has been imported. mypy_primer reports no type-check changes on the corpus. This completes closing triple-quoted strings in the LSP. Could a maintainer review when you have a moment?

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅