Add fourslash tests for importModuleSpecifier user preferences

[Copilot]

Implements fourslash tests for importModuleSpecifierPreference and importModuleSpecifierEnding user preferences added on the auto-imports-user-prefs branch.

Changes

Fixed import cycles blocking build

• Changed ls.UserPreferences → lsutil.UserPreferences in findallreferences.go and server.go

Added 7 fourslash tests

• importModuleSpecifierPreference: shortest, relative, non-relative, project-relative (4 tests)
• importModuleSpecifierEnding: minimal, index, js (3 tests)

Each test configures user preferences, verifies completions include expected imports, and generates baselines showing the applied preference:

f.VerifyCompletions(t, "", &fourslash.CompletionsExpectedList{
    UserPreferences: &lsutil.UserPreferences{
        ImportModuleSpecifierPreference: modulespecifiers.ImportModuleSpecifierPreferenceNonRelative,
    },
    Items: &fourslash.CompletionsExpectedItems{
        Includes: []fourslash.CompletionsExpectedItem{"helperFunc"},
    },
})
f.BaselineAutoImportsCompletions(t, []string{""})

Generated baselines capture actual import paths with preferences applied (e.g., "src/utils/helper" for non-relative vs "./utils/helper" for relative).

Not Included

Additional preferences deferred for future work: autoImportFileExcludePatterns, quotePreference, allowRenameOfImportPath, completion snippet preferences, organizeImports preferences.

Original prompt

Can you implement fourslash tests for the user preferences that were added on this branch?

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