fix: serialize tool-only Chat Completions content

[grikomsn]

Summary

• serialize textless Chat Completions messages with content: "" instead of null
• preserve existing text/image content-part behavior
• add regression coverage and a patch changeset

Fixes #8

Verification

• npm run check — 71 tests passed
• npm run package — VSIX packaging passed
• sideloaded 0.4.0 into VS Code 1.134.0
• live OpenCode Go inference succeeded with MiniMax-M3

The current VS Code profile exposes the Go credential through the extension’s management command rather than as a selectable Go chat-model entry, so the exact DeepSeek tool-roundtrip was validated through the focused serialization regression test.

[pullfrog]

✅ No new issues found.

Reviewed changes This review covers the shared content normalization, downstream request serialization, regression tests, and changeset; local npm test passes all 71 tests.

• Content normalization — textless Chat Completions messages now serialize with an empty string while existing text and image content-part shapes remain unchanged.
• Provider integration — the shared type/helper refactor is compatible with the Anthropic and Google serializers, while the Responses path remains separate.
• Regression coverage and release metadata — focused tests cover empty and image content plus a tool-only assistant request, and the patch changeset is present.

  ^{｜ View workflow run ｜ Using GPT Luna ｜ 𝕏}