[Refactor] model family resolution to support non-prefixed names like Mistral

[SongChiYoung]

Why are these changes needed?

This PR improves how model_family is resolved when selecting a transformer from the registry.
Previously, model families were inferred using a simple prefix-based match like:

if model.startswith(family): ...

This works for cleanly prefixed models (e.g., gpt-4o, claude-3) but fails for models like mistral-large-latest, codestral-latest, etc., where prefix-based matching is ambiguous or misleading.

To address this:
• model_family can now be passed explicitly (e.g., via ModelInfo)
• _find_model_family() is only used as a fallback when the value is "unknown"
• Transformer lookup is now more robust and predictable
• Example integration in to_oai_type() demonstrates this pattern using self._model_info["family"]

This change is required for safe support of models like Mistral and other future models that do not follow standard naming conventions.

Related issue number

Linked to discussion in #6151
Related : #6011

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

Attention: Patch coverage is 95.45455% with 1 line in your changes missing coverage. Please review.

Project coverage is 77.33%. Comparing base (9143e58) to head (8397958).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...n-ext/src/autogen_ext/models/openai/_model_info.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6158      +/-   ##
==========================================
+ Coverage   77.32%   77.33%   +0.01%     
==========================================
  Files         197      197              
  Lines       13797    13808      +11     
==========================================
+ Hits        10669    10679      +10     
- Misses       3128     3129       +1     

Flag	Coverage Δ
unittests	77.33% <95.45%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[SongChiYoung]

@ekzhu First of all, Done.

🙋‍♂️ Should fallback raise branches be covered by tests for Codecov?
Just in case, I added tests — but want to confirm if it’s required for future PRs.
(However I could not add warnings.warn test case, So, if we need to test it, we need to add to _MODEL_INFO only for testcase)
⸻

⚠️ Saw a Codecov GPG signature error.
Pretty sure it’s not from my side 😅 — just checking!

[SongChiYoung]

@ekzhu Sorry… Before the merge, there was no formatting error on my PC, but after the merge, one error was added (automatically resolved).
I didn’t notice it because it was auto-resolved silently. ;(

So it’s now resolved.

Ehhhhhh… Thank you for fixing it!
When I pushed my code, I realized that you had already fixed it.

⸻

FYI: My typical contribution window is KST 9PM–2AM and 6AM–9AM.
I may be slower to respond during KST 9AM–7PM, except for urgent issues.
Just wanted to share this for better async coordination. 🙇