Fix LSP kind for Jedi string completions

[AryanRG]

Fixes #472.

Jedi reports dictionary-key completions with type string. The existing type map emits a present completion kind of null; LSP defines a present kind as an integer enum. Map string completions to CompletionItemKind.Value without changing labels or insertion text.

Validation:

• Two new regression cases fail on the pristine base and pass with this change.
• Python 3.9 with Jedi 0.17.2: 9 focused tests passed.
• Python 3.12 full local suite: 201 passed, 10 skipped, 2 failed. The same two failures (PyQt and Matplotlib completion cases) also occur on the untouched base, where Jedi returns no completions.
• Ruff check and format check passed.

AI assistance was used to investigate, implement, test, and review this change.