fix(opencode): surface missing server plugin entrypoints

[kvnloo]

Issue for this PR

Closes #48577

Type of change

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

What does this PR do?

The server plugin loader currently leaves "report.missing" empty, so a configured plugin with no server entrypoint can disappear without a useful error.

Route that case through the existing "publishPluginError" path. TUI plugin handling is unchanged.

Based on #48699 by @RaviTharuma, with related work in #44372 by @AllanSantos-DV.

How did you verify your code works?

"cd packages/opencode && bun test test/plugin/missing-entrypoint.test.ts"

The tests cover missing server entrypoints, the existing TUI path, and the production "report.missing" wiring.

Screenshots / recordings

N/A

Checklist

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

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[github-actions]

Hey! Your PR title Factory/plugin missing strengthen 48699 doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

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[github-actions]

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