[Refactor] Reuse shared CustomModesManager test helpers

[zoomote]

Created by Roomote. Follow up by mentioning @roomote, in the web UI, or in Discord.

What changed

• Reused the shared VS Code extension context and URI factories across the three CustomModesManager config specs.
• Reused the shared mock reset helper for suite teardown.
• Kept each spec's filesystem, export/import, watcher, and YAML edge-case setup local to its scenarios.

Why this change was made

This continues the incremental test-utils rollout with a small, mechanical config-test batch. It removes repeated VS Code test scaffolding without extracting behavior-specific filesystem fixtures or widening the scope to the larger import/export suite.

Impact

Test-only refactor. CustomModesManager behavior and coverage are unchanged, while the config specs now share the established test utility conventions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[zoomote]

No code issues found.

_{Reviewed c71c059}