convert: use existing local chat_template if mistral-format model has one. (#17749)

convert: use existing local chat_template if mistral-format model has one. (#17749)

* conversion: use existing local chat_template.jinja file if mistral-format model has one.

* fix --mistral-format mistakenly assuming some <=v7 chat template names are file paths and reading them.

* Update convert_hf_to_gguf.py - change from exists() to is_file()

Co-authored-by: Sigbjørn Skjæret <redacted>
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Co-authored-by: Sigbjørn Skjæret <redacted>