fix provide auto completion when match Literal #2178

[asukaminato0721]

Summary

Fixes #2178

Added match-pattern literal completions by detecting PatternMatchValue/PatternMatchSingleton at the cursor and using the match subject’s inferred type to surface Literal values.

Test Plan

A regression test for string literal matching.

[Copilot]

Pull request overview

This PR adds auto-completion support for literal values in Python match statements. When typing a case pattern for a variable with a Literal type annotation, the LSP now suggests the possible literal values.

Changes:

• Added add_match_literal_completions function to detect match patterns and provide completions based on the subject's type
• Integrated the new completion function into the existing completion flow
• Added a regression test for string literal matching

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
pyrefly/lib/state/lsp.rs	Added add_match_literal_completions method that detects PatternMatchValue and PatternMatchSingleton nodes, retrieves the match subject's type, and provides literal completions
pyrefly/lib/test/lsp/completion.rs	Added test case completion_literal_match_value to verify literal completions work correctly in match statements

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[github-actions]

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