[codex] Preserve system prompts for no-system templates

[mimeding]

Summary

This moves the Gemma/system-prompt fix to the upstream dependency instead of keeping an Osaurus-side shim for osaurus-ai/osaurus#992.

• Preserve system instructions for templates that cannot accept system roles by folding them into the first user message, or inserting a synthetic user turn when needed.
• Keep tool calls intact through NoSystemMessageGenerator.
• Make a clean public checkout resolvable when the ignored local RunBench/ directory is absent.
• Repair the Osaurus macOS CI path by using the vmlx-swift-lm-Package scheme and the matching DerivedData test bundle path.
• Serialize the MLX sampling portions of EvalTests to avoid Metal command-encoder contention during the full Xcode test bundle.

Validation

• git diff --check
• swift package describe --type json
• swift test --filter ChatMessageToolCallTests
• xcodebuild build-for-testing -scheme vmlx-swift-lm-Package -destination 'platform=macOS'
• xcrun xctest ~/Library/Developer/Xcode/DerivedData/vmlx-swift-lm-exec-*/Build/Products/Debug/MLXLMTests.xctest
  • 512 tests passed locally; Gemma4 local snapshot tests skipped because the optional model snapshots are not installed.

Note: direct SwiftPM execution of the Metal-heavy EvalTests still hits the local default.metallib loading limitation, so the full validation path uses the Xcode-built test bundle, matching the macOS workflow.

[mimeding]

Maintainer/review note for this PR:

This PR is the upstream replacement for the Osaurus-side shim in osaurus-ai/osaurus#992. @tpae correctly pointed out that the fix belongs in vmlx-swift-lm, not in Osaurus as a Gemma-specific compatibility case.

What I need from maintainers here:

• Review the NoSystemMessageGenerator behavior: system instructions are preserved by folding them into the first user turn for templates that cannot accept a system role, while tool calls remain intact.
• Confirm whether this is the preferred upstream behavior for no-system templates, or whether you want the folding format adjusted.
• Check/enable CI for this repo. The workflow is active, but GitHub has not attached checks to this PR yet. This branch includes a workflow fix for the stale mlx-swift-lm-Package scheme and DerivedData path, plus package-resolution fixes needed in a fresh public checkout.

Local validation is green:

• git diff --check
• swift package describe --type json
• swift test --filter ChatMessageToolCallTests
• xcodebuild build-for-testing -scheme vmlx-swift-lm-Package -destination 'platform=macOS'
• full Xcode-built MLXLMTests.xctest: 512 tests passed locally

Once this lands, the Osaurus follow-up should be only a dependency bump plus a small regression test, and osaurus-ai/osaurus#992 can be closed or replaced.