Add inlay hints for tuple unpacking variables (#2096)

[jackulau]

Summary

Adds support for displaying inlay hints on variables created by tuple unpacking.

Before: Variables from tuple unpacking like x, y = (a, b) showed no inlay hints.

After: Each unpacked variable shows its individual type hint:

• x: Literal[1]
• y: Literal[1]

Changes

• Handle Binding::UnpackedValue in inlay hints generation (inlay_hints.rs)
• Add helper function get_unpacked_element_expr() to extract element expressions from tuple/list literals
• For nested unpacking or function calls where element expressions can't be extracted, fall back to showing hints based on type information
• Skip hints for literal values (e.g., x, y = (1, 2) shows no hints)
• Respect existing annotations (no hints when variables are already annotated)

Test plan

Added 7 new test cases

• test_tuple_unpacking_inlay_hint - Basic tuple unpacking: x, y = (a, b)
• test_tuple_unpacking_from_function_call - Unpacking from function calls: x, y = f()
• test_tuple_unpacking_no_hint_for_literals - No hints for literals: x, y = (1, 2)
• test_tuple_unpacking_with_prior_annotation - Prior annotations respected
• test_nested_tuple_unpacking - Nested unpacking: (a, b), c = f(), 3
• test_starred_unpacking_from_function - Starred unpacking from function
• test_starred_unpacking_from_literal - Starred unpacking from literal

[github-actions]

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

[stroxler]

Thanks for the PR! This looks good to me, I'll try to get a second review + merge

[meta-codesync]

@stroxler has imported this pull request. If you are a Meta employee, you can view this in D90653949.