fix(provider): keep compacted Anthropic tool histories user-led

[codeg-dev]

Issue for this PR

Closes #31048

Type of change

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

What does this PR do?

Normalizes Anthropic-bound message histories in two narrowly scoped cases:

1. Strips trailing assistant prefill for affected Claude 4.6/4.7/4.8 model IDs that reject assistant-final requests.
2. Prepends a minimal synthetic user boundary when a compacted Anthropic history starts with an assistant tool-call message and the immediately following tool message satisfies every leading tool call.

This keeps Anthropic request histories user-led where required while leaving non-Anthropic providers unchanged. Assistant tool-call prefill is preserved when thinking mode still needs it.

Prepared with AI assistance.

How did you verify your code works?

• git diff --check -- packages/opencode/src/provider/transform.ts
• Built a v1.16.2 custom artifact from the same transform change plus the already-open session processor patch.
• Deployed the artifact to private host, private host, private host, and codeg-hq.
• Verified openai/gpt-5.5 normal smoke and tool-failure recovery on all custom hosts; hq also verified the Windows desktop backend /, /agent, and /config returned 200.

Screenshots / recordings

Not a UI change.

Checklist

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

[github-actions]

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

[codeg-dev]

Rebased onto latest dev and force-pushed. CI is green (check-compliance ✅, check-standards ✅). This PR fixes #31048 — ready for review whenever you have a moment. Thanks!

[github-actions]

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