fix(openai-shim): include text with image-only content

[MikeTomlin19]

Summary

• Ensure OpenAI-compatible multipart image payloads always include a non-empty text part.
• Preserve existing mixed text+image payloads while adding coverage for image-only user and tool-result paths.

Why

Some OpenAI-compatible providers, including the Xiaomi MiMo/OpenGateway reports discussed around #425, reject image requests with text is not set when the content array contains only image_url parts.

Refs #425.

This is intentionally scoped to the MiMo/OpenGateway image-capable compatibility case. It does not attempt to solve text-only provider image handling, which is tracked separately by #1382/#1391.

Test plan

• bun test src/services/api/openaiShim.test.ts
• bun test src/services/api/openaiShim.test.ts src/utils/model/modelOptions.xiaomi-mimo.test.ts src/services/api/providerConfig.local.test.ts
• bun run build
• bun run smoke
• git diff --check
• GitHub web check
• GitHub smoke-and-tests reached the full suite and failed only on the same 4 unrelated tests already failing on latest main PR Checks run 26548651163
• bun test locally (3005 pass / 10 fail; failures are unrelated existing attribution, auto-compact, breaker metadata, and provider model env tests)
• [blocked] bun run typecheck (repo-wide generated/missing-module/type errors unrelated to this patch; first failure is src/QueryEngine.ts missing ./services/compact/snipProjection.js)

[Meetpatel006]

can you check for the ci faling issue

[MikeTomlin19]

Checked it. The failing check is the full smoke-and-tests suite, not the targeted image payload coverage from this PR.

The run failed on these 4 tests:

• getAttributionTexts > preserves includeCoAuthoredBy true as an explicit old-default opt-in
• active auto-compact cooldown blocks before model call with cooldown guidance
• auto-compact cooldown tracking is carried into the next query call
• breaker metadata tracking callback publishes a fresh object

Those are outside this PR's OpenAI shim/image-only content path. This PR's targeted tests and build/smoke passed locally, and the separate web check is green here. I also see main had a PR Checks failure around the same window, then a later main PR Checks run is green, so this looks like stale base/CI state rather than something introduced by this patch.

A rerun after the current main state should be the right next check.