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

[matchaxnb]

Issue for this PR

Closes#34321
This supersedes #34322 by @MADEVAL because some further fix was missing (collapse again in the messages)

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 #34322 - fix(opencode): collapse plugin-pushed system messages into one
#34322

Relationship: This is the exact same fix that PR #38671 supersedes. According to the current PR's description, #38671 addresses the same issue as #34322 but with an additional correction (collapse guard fix that was missing in the previous attempt).

Since the current PR explicitly states it "supersedes #34322 by @MADEVAL," these are directly related—#34322 should likely be closed in favor of #38671.

[ondrejch]

Preparing a narrower replacement in #42801.

#38671 merges adjacent system messages for every provider. #42801 only coalesces them for @ai-sdk/openai-compatible, so Anthropic can keep separate system segments for prompt-cache boundaries.

[matchaxnb]

Oka closing, cheers!