fix(health): quiet checkhealth for unused VCS tools and optional icons

[dlyongemallo]

Relates to #316.

[Copilot]

🟢 Approval recommended

The change is small, self-contained, behaviorally correct, and has no lingering references to the removed symbols.

Pull request overview

This PR reduces :checkhealth diffview noise for users who don't have every optional dependency installed, addressing issue #316. Previously, checkhealth warned about each missing icon plugin and every uninstalled VCS tool (jj, hg, p4). The updated logic treats the two icon plugins as interchangeable alternatives (info-level when none is present) and only surfaces VCS warnings/errors when no supported VCS tool is valid.

Changes:

• Replace the plugin_deps list (with essential/optional handling) with icon_providers, reporting info instead of a warning when no icon provider is found.
• Buffer VCS adapter results into OK/failed buckets and return early once any adapter is valid, so unused VCS tools no longer produce warnings.
• Remove the now-unused system_list helper and the missing_essential gating.

File summaries

File	Description
lua/diffview/health.lua	Rework icon-provider check to info-level and quiet VCS warnings when at least one adapter is valid; remove unused helpers.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Balanced

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