A .gguf whose filename holds no recognizable quant token still gets a label
from the scanner, which falls back to the filename stem, so the UI stores keys
like "/models/custom.gguf:custom". _bare_model_id accepted only known quant
tokens, so the first per-quant save did not carry over llama_extra_args stored
under the bare id, and auto-switch prefers the qualified entry, so those flags
were silently dropped with no UI able to restore them.
Accept a suffix that is exactly the label the scanner derives for that filename.
Requiring the head to be a .gguf and the suffix to match exactly is what keeps
an arbitrary colon-containing POSIX path out: "/models/foo:bar.gguf" splits to
a head that is not a .gguf. The filename is taken by splitting on both
separators, since a "C:\\..." key is written on Windows but may be read back
by a backend that is not.
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