fix(copilot): treat synthetic user messages as agent for x-initiator billing

[Juhwa-Lee1023]

Issue for this PR

Closes #15824

Originally reported upstream at #8030.

Type of change

• Bug fix

What does this PR do?

When GitHub Copilot receives a request, it uses the x-initiator header to decide whether to bill the request against the user's premium quota (user) or the agent budget (agent). opencode sets this header based on the role of the last message in the request body.

The problem: tool results, image attachments, and other agent-produced content travel as messages with role: "user" in all three API flavours (Completions, Responses, Anthropic Messages). The Completions and Responses API paths only checked last?.role !== "user", so every agentic turn containing tool results was mis-classified as user-initiated and incorrectly billed against the user's Copilot premium quota.

The Anthropic Messages API path had a partial fix (hasNonToolCalls), but it was inconsistent with the other two paths and only covered tool_result parts.

Fix: introduce a shared isSyntheticUserMessage() helper that returns true when every content part of a role: "user" message is an agent-produced type (tool_result, attachment, input_image, image_url, image). Apply it consistently across all three API paths.

How did you verify your code works?

Added 13 unit tests in packages/opencode/test/provider/copilot/x-initiator.test.ts covering genuine human messages (→ false) and all synthetic agent-produced message shapes (→ true).

bun test test/provider/copilot/
42 pass, 0 fail

Screenshots / recordings

N/A — logic-only change, no UI.

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:

I found a potentially related PR:

Related PR:

• fix: Prevent excessive Copilot premium request consumption #8721 - "fix: Prevent excessive Copilot premium request consumption"
  • fix: Prevent excessive Copilot premium request consumption #8721
  • This appears to be addressing the same billing issue where agent-produced content (tool results, images, attachments) was incorrectly consuming Copilot premium quota instead of agent budget.

This PR (15821) may be a more comprehensive fix or refactor of the issue originally addressed in #8721. You should verify whether #8721 was closed in favor of this new PR or if they address different aspects of the same problem.

[github-actions]

Thanks for your contribution!

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[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[Juhwa-Lee1023]

Closing in favour of #8721 which provides a more complete fix using history-based inference (matching VSCode Copilot's iterationNumber === 0 approach) plus synthetic text pattern detection. Our content-based approach here is a subset of that work. #8721 also tracks the long-term metadata-based solution in #8766.