fix(opencode): collapse plugin-pushed system messages into one

[MADEVAL]

Issue for this PR

Closes #34321

Type of change

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

What does this PR do?

When a plugin uses experimental.chat.system.transform to push additional system instructions, the collapse guard system.length > 2 was one too high. The initial array has 1 element and a plugin that pushes 1 entry makes it 2, so 2 > 2 never fired. The result was multiple {role: "system"} messages, which OpenAI-compatible providers reject.

The fix changes the guard from > 2 to > 1. The system[0] === header check still protects against intentional header replacement by plugins.

How did you verify your code works?

Added a test in transform.test.ts that simulates a plugin pushing to system and verifies the prepared messages array has exactly one system message containing the extra instructions. Traced the full code path: system array construction, plugin trigger, collapse guard, and message construction.

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:

Potential Duplicate Found

PR #34267: fix(llm): collapse system messages when plugin appends a single entry
#34267

This appears to be addressing the same issue—collapsing multiple system messages when plugins append entries. This PR likely covers the same fix for handling plugin-pushed system instructions.

[matchaxnb]

affected by this too.

[github-actions]

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