fix(api): return 404 for unknown weight-file model names instead of gallery fallthrough

[localai-bot]

What

Tightens the model-existence guard in SetModelAndConfig so an unknown model name that points at a concrete weight file returns a clean 404 instead of silently falling through to the gallery autoloader and triggering a surprise HuggingFace download.

Fixes #10162.

Why

The guard skipped the existence check for any model name containing /, to let HuggingFace org/repo IDs (which backends like diffusers download on the fly) pass through. But a name like local/model.gguf (the parameters.model weight path, mistakenly passed as the request model) also contains /, so it bypassed the guard. With AUTOLOAD_GALLERIES on by default, the request then fell through to the gallery autoloader, which attempted to download an unrelated GGUF from HuggingFace instead of returning a clear error.

In the linked issue, a user passed --model local/DeepSeek-R1-Distill-Qwen-1.5B-BF16.gguf (the file path) rather than the configured YAML name, and saw LocalAI try to download from bartowski/... instead of failing fast.

How

• Only treat a /-containing name as a remote org/repo ID when it does not end in a recognized model-file extension. Names pointing at a concrete file are verified like any other.
• New model.HasKnownModelFileExtension reuses the existing knownModelsNameSuffixToSkip list as the single source of truth. Version-style suffixes (e.g. the .0 in stabilityai/stable-diffusion-xl-base-1.0) are not in that list, so genuine repo IDs are still treated as remote and pass through.
• CheckIfModelExists already resolves relative paths against the models dir, so a loose weight file addressed by its relative path (and that genuinely exists) still passes.

Tests

• core/http/middleware/request_test.go: a missing weight-file path (local/missing-model.gguf) now returns 404; an existing one (local/present-model.gguf) passes through. Existing HuggingFace-ID passthrough test (stabilityai/stable-diffusion-xl-base-1.0) stays green.
• pkg/model/loader_test.go: unit tests for HasKnownModelFileExtension (file paths vs repo IDs).

Behavior unchanged for legitimate HuggingFace IDs and for AUTOLOAD_GALLERIES when a real gallery match exists.