Fix first plugin skill completion

[Paul (pwang347)]

Summary

• route agent-host skill completion discovery through the existing serialized session customization publisher
• ensure SessionCustomizationsChanged is emitted before the completion response can reach the workbench
• add a regression test covering a newly discovered custom plugin skill

Fixes #333203

Verification

The regression test demonstrates the ordering bug directly: before the fix the skill completion is returned, but SessionState.customizations is still undefined.

Without the fix

AgentService (node dispatcher)
  1) publishes a newly discovered skill before returning its completion

0 passing
1 failing

actual:
  items: ['/dummy:print-hello-world ']
  published: undefined

With the fix

AgentService (node dispatcher)
  ✔ publishes a newly discovered skill before returning its completion

1 passing

Related tests

AgentHostSkillCompletionProvider: 19 passing
AgentSideEffects customization tests: 16 passing

UI verification

Launched Code - OSS Dev from this PR branch with the minimal plugin from #333203 and a fresh Copilot chat. On the first attempt, completion returned /dummy:print-hello-world; accepting it immediately produced the recognized blue skill token shown below. The DOM assertion independently confirmed ced-chat-dynamic-variable-1 and ced-chat-session-clickable-text-1 on the token with color rgb(133, 182, 255).

[Copilot]

Copilot review overview

🟢 Approval recommended

The focused ordering fix is consistent with existing publication logic and has direct regression coverage.

Review tier: Balanced
Findings: None

What changed in this PR

Routes skill completion discovery through serialized customization publishing, ensuring new skills reach session state before completion results.

Changes:

• Exposes an awaitable customization refresh path.
• Uses it for skill completions.
• Adds regression coverage for first-use plugin skills.

File	Description
agentSideEffects.ts	Returns resolved customizations from serialized publication.
agentService.ts	Connects completions to the publisher.
agentHostSkillCompletionProvider.ts	Supports injected customization resolution.
agentService.test.ts	Verifies publication precedes completion.

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