fix: For non-anthropic providers, combine system prompts (allows Qwen 3.5 to work)

[paulgear]

This is an attempted fix for issue #5034 which works for me using Qwen3.5-35B-A3B.

Issue for this PR

(Hopefully) Closes #5034

Type of change

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

What does this PR do?

When I updated my llama.cpp model from Qwen3-Coder-30B-A3B-Instruct to Qwen3.5-35B-A3B I started receiving the error message noted in my comment on #5034. I asked Sonnet 4.5 to analyse it and it suggested two possible fixes, one which unconditionally combines system prompts, and another which does it only for providers other than Anthropic. I chose the latter. I can provide the prompting artifacts if that's desirable.

How did you verify your code works?

• I ran the typecheck.yml and test.yml workflows against my own branch
• I built an opencode binary which I tested against my llama.cpp setup using Qwen3.5-35B-A3B-UD-Q6_K_XL from https://huggingface.co/unsloth/Qwen3.5-35B-A3B-GGUF. This did not produce the error and seems fully functional as a model for OpenCode, whereas without this fix it gives HTTP 500 errors and doesn't do anything useful at all.

Screenshots / recordings

n/a

Checklist

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

[github-actions]

Hey! Your PR title For non-anthropic providers, combine system prompts doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[paulgear]

@thdxr & @rekram1-node Apologies to bother you with direct mentions, but you seem to be the most common contributors to this file. Is there any chance of some eyes on this PR? Qwen3.5 and some other models seem unusable with OpenCode without it.