fix: model path backward compatible

[louis-jan]

Describe Your Changes:

• This is to address the issue where old versions (less than 0.4.4) generate GGUF model file names as model.id. Recent updates will only look for GGUF filename. It now scans for the GGUF/Model ID binary file.
• Also, support absolute model file paths (place it under ~/Downloads or somewhere else).

Scenario 1: Model file name the same as model id (<= 0.4.4)

Scenario 2: Model file name is the same as downloaded GGUF file (<= 0.4.6)

Scenario 3: Model binary can be anywhere

Fixes Issues

• bug: Model - New model.json causing unable to load old model downloaded from Hub, needs modification / redownloading  #1922
• feat: users can symlink model binaries #1381

Self Checklist

• Added relevant comments, esp in complex areas
• Updated docs (for bug fixes / features)
• Created issues for follow-up changes or refactoring needed