fix(opencode): ensure tool_use/tool_result integrity and Anthropic user-first ordering

[TeddyEngel]

Issue for this PR

Closes #27594

Type of change

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

What does this PR do?

(The PR implements fix 4 from the linked issue: "A defensive pass that pairs every tool_use with the matching tool_result before dispatch.")

Sessions that had undergone nested compaction would fail to resume. The specific error encountered was from Anthropic:

messages.2: `tool_use` ids were found without `tool_result` blocks immediately after

Two issues were identified:

Orphan Tool-Calls (All Providers)

After nested compaction, tool-calls could become orphaned (no corresponding tool-result). Most LLM APIs reject this.

User-First Ordering (Anthropic Only)

After nested compaction, the message array could start with system messages followed directly by an assistant message. Anthropic requires the first non-system message to be user:

How did you verify your code works?

• All 276 tests pass
• New tests cover the edge cases identified.
• I tested resuming a session that had crashed with that dead-end case - the session was succesfully restored with the fix.

Checklist

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

If you do not follow this template your PR will be automatically rejected.

[rekram1-node]

believe this has previously been fixed elsewhere actually, unless u can send me a session where u hit it

[TeddyEngel]

@rekram1-node it's unfortunately still an issue. Here is a redacted session that crashes on latest dev, resumes ok with my fix: crash_minimal.json