Vision: Keep colons in OpenAI-compatible model IDs #5863 - #5872

Description

With Engine: openai, GetModel split the configured identifier on the first colon and sent only the part before it. IDs like unsloth/Qwen3.5-9B-GGUF:Q4_K_M or OpenAI fine-tunes (ft:gpt-4o-mini-...) reached the server truncated, so every request failed with model not found.

OpenAI-compatible servers match the ID verbatim, so GetModel now returns it unchanged for that engine. Ollama and the other engines still split name:version as before. I added two cases to TestModel_GetModel and updated the vision README.

I tested this in the photoprism/develop:260921-resolute image after make dep-models. go test ./internal/ai/vision/... and go test ./internal/commands -run Vision pass, gofmt and goimports are clean, and golangci-lint reports nothing on the changed lines. With the vision.yml from the issue, photoprism vision ls now lists the model as unsloth/Qwen3.5-9B-GGUF:Q4_K_M.

One thing I left alone: openai.IsCloudModel does not look past the ft: prefix, so a fine-tuned GPT model still gets the self-hosted defaults. Happy to follow up if you want that handled too.

Related Issues

Fixes #5863

Acceptance Criteria

• New features or enhancements are fully implemented and do not break existing functionality, so that they can be released at any time without requiring additional work
• Automated unit and/or acceptance tests are included to ensure that changes work as expected and to reduce repetitive manual work
• Documentation has been / will be updated, especially as it relates to new configuration options or potentially disruptive changes