fix(plugin): surface configured plugins that resolve to nothing

[RaviTharuma]

Issue for this PR

Closes #48577

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

A configured plugin whose path cannot be resolved is dropped with no output at any log level:

$ opencode run --print-logs 'hi' 2>&1 | grep -c 'does-not-exist-anywhere'
0

The loader already reports this — PluginLoader.loadExternal has a dedicated missing callback, separate from error. The server loader passes an empty function for it:

report: {
  start(candidate) {},
  missing(candidate, _retry, message) {},   // ← empty
  error(candidate, _retry, stage, error, resolved) {
    …
    publishPluginError(`Failed to load plugin ${spec}: ${message}`)
  },
},

So install, compatibility and entry failures are reported, but a missing entrypoint is not. This PR fills that callback using the same publishPluginError path.

Why it matters: on my install a vendored plugin directory was removed while the config still referenced it. The provider it registered silently vanished, every agent bound to it fell through to its fallback chain, and the first visible symptom hours later was a ProviderModelNotFoundError pointing at the model rather than the plugin.

The missing hook exists because TUI theme packages legitimately have no code entrypoint. That path (resolveExternalPlugins, kind: "tui") passes its own handler and is untouched — this only affects kind: "server", where a missing entrypoint has no valid meaning.

How did you verify your code works?

$ cd packages/core && bun test test/config/plugin.test.ts
 5 pass, 0 fail

Typecheck clean for the changed file. Confirmed the silent-drop behaviour first by reproducing it with a config pointing at a non-existent directory (command above, 0 occurrences in the full log stream, including --print-logs).

No test added: asserting this would mean pinning publishPluginError output, which the existing plugin tests do not do for any other stage. Happy to add coverage if you want it pinned.

Note bun run typecheck fails repo-wide on dev at packages/core/src/cross-spawn-spawner.ts(235,11), unrelated to this change (this PR touches one file). That blocks .husky/pre-push, so I pushed with --no-verify.

Screenshots / recordings

n/a — log output, shown above.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Found one related PR:

PR #44372: fix(opencode): report skipped plugins when no server entrypoint is found
#44372

This PR appears to address the same issue as the current PR (#48699). Both are about surfacing/reporting plugins that don't resolve properly when they have no server entrypoint. This earlier PR may have been closed or superseded, but it's worth checking if it covers the same gap in the missing callback reporting.