fix(plugin): surface missing server plugin entrypoints (origin #48699)

[kvnloo]

Summary

Surface configured server plugins that resolve to no server entrypoint instead of silently dropping them.

The server loader's report.missing callback now routes through the same visible plugin-error path used by other load failures. Focused coverage exercises missing server and TUI entrypoints and pins the server wiring.

Credit / provenance

This builds directly on upstream anomalyco#48699 by @RaviTharuma, who identified the silent report.missing seam and proposed the production fix. Earlier related entrypoint-reporting work in anomalyco#44372 by @AllanSantos-DV also helped establish the problem space. This branch refreshes the fix and adds focused regression coverage.

Current refresh

• rebased cleanly onto current dev on 2026-10-03
• 0 commits behind
• changes only packages/opencode/src/plugin/index.ts plus one focused test file
• no unrelated changes

github_writes_origin=0 — ready to promote when upstream write access is available.

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