Fix/ollama system prompt

[wiizzl]

🎯 Changes

Fixes #567

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm run test:pr.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).

Summary by CodeRabbit

Bug Fixes

• System prompt handling in the Ollama chat adapter has been refined to ensure proper integration with chat requests. System prompts are now consistently processed within the chat API workflow, improving message handling, consistency, and overall chat operation reliability.

[coderabbitai]

📝 Walkthrough

Walkthrough

This PR fixes the Ollama adapter's handling of global system prompts by moving them from a dedicated request field into the messages array as a leading system message, aligning with Ollama's chat endpoint expectations. A changeset entry documents the patch release.

Changes

System Prompts in Ollama Chat API

Layer / File(s)	Summary
System prompts prepended to messages array packages/typescript/ai-ollama/src/adapters/text.ts, .changeset/strong-heads-hang.md	mapCommonOptionsToOllama now stores formatted messages, prepends a role: 'system' message with joined systemPrompts when present, and returns the request with the updated messages list instead of a separate system field. Changeset documents the patch-level chat API behavior change.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

🐰 A whisper in the messages flow,
System prompts now lead the way,
No longer lost in fields below,
Ollama listens what we say! 🎉

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'Fix/ollama system prompt' clearly and concisely summarizes the main change: fixing how the Ollama adapter handles system prompts.
Linked Issues check	✅ Passed	The code changes fully implement the proposed fix from issue #567: system prompts are now prepended as a system message to the messages array instead of placed at root level, matching Ollama's chat API expectations.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to the reported issue: the text adapter is modified to fix system prompt handling, and a changeset is added for the release, with no unrelated modifications present.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description check	✅ Passed	The PR description includes the required template sections with checked boxes indicating contributor completed the contributing guide and generated a changeset for release impact.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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