fix(import): derive model name from selected GGUF for repo-root URIs

[Dennisadira]

What

Fixes #10587.

When importing a HuggingFace GGUF model from a repository-root URI (no file component, e.g. hf://owner/repo) with the Model Name field left blank, the importer named the model after the repository (filepath.Base(details.URI)) instead of the GGUF file it actually selected from the repo listing. The emitted name:, the model: path, and the gallery Filename directory all inherited the repo name rather than the model.

Fix

• Track whether the user supplied an explicit name (nameProvided); the URI base is now only a fallback.
• In the HuggingFace branch, once the preferred model group is picked, re-derive the name from the selected GGUF via a new modelNameFromShardGroup helper that uses ShardGroup.Base minus the .gguf extension.
  • For sharded models this yields a clean logical name (e.g. Qwen3-30B-A3B-Q4_K_M) rather than a shard filename like ...-00001-of-00002.
  • For single-file repos it's the file basename minus extension.
• An explicit name preference still always wins. The .gguf / URL / OCI import paths are unchanged.

Tests

• New network-free unit specs in llama-cpp_test.go: name-from-single-GGUF, clean-name-from-shard-base, and explicit-name precedence.
• Updated the live integration specs in importers_test.go that had encoded the previous repo-name behaviour to assert the GGUF-derived name.

All importer specs pass; go vet and gofmt clean.

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