Strip matching prefix in path mapping completions

[gabritto]

Fixes #3981.

Nice to notice Strada didn't have this problem because this scenario was just completely broken then. 😅

[Copilot]

Pull request overview

Fixes an issue in the language service’s module-specifier/path-mapping completion logic where completion items could repeat already-typed path components (e.g. suggesting unstable/ast when completing under .../unstable/, leading to unstable/unstable/... after commit).

Changes:

• Strip the already-typed fragment directory prefix from path-mapping completion item names when the completion target already includes the fragment.
• Add fourslash regression tests covering partial-path completions for relative imports and package deep-imports (with and without exports, including wildcard patterns).

Reviewed changes

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

File	Description
internal/ls/string_completions.go	Adjusts path-mapping completion name shaping to trim the fragment directory prefix and avoid duplicated path components.
internal/fourslash/tests/pathCompletionsPartialPath_test.go	Adds regression tests verifying expected completion items for partial-path module specifiers across several scenarios.