fix: fallback custom model maxOutputTokens to OUTPUT_TOKEN_MAX

[MertSoylu]

Issue for this PR Closes #22253 ### Type of change - [x] Bug fix ### What does this PR do? Fixes custom provider models failing with "maxOutputTokens must be >= 1" when limit field is not defined. The issue was in provider.ts custom model parsing where models without a limit field fell back to 0 instead of OUTPUT_TOKEN_MAX (32000). Changed the fallback to use ProviderTransform.OUTPUT_TOKEN_MAX, consistent with built-in models. This is a different approach from PR #22016 which fixed the same issue in the transform.ts maxOutputTokens() function. This fix addresses it at the provider config parsing level. ### How did you verify my code works? - Analyzed the code to understand the fallback logic - Confirmed built-in models use OUTPUT_TOKEN_MAX as fallback - Changed custom model parsing to use the same fallback ### Checklist - [x] I have tested my changes locally - [x] I have not included unrelated changes in this PR

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Found a related PR:

PR #22016 - "fix: handle invalid maxOutputTokens value (0 or undefined)"

• fix: handle invalid maxOutputTokens value (0 or undefined) #22016

This PR appears to be closely related as it also addresses the maxOutputTokens validation issue. Both PRs are fixing the "maxOutputTokens must be >= 1" validation error, though PR #22016 may be handling the issue differently (dealing with 0 or undefined values). This could be a duplicate or overlapping fix for the same underlying issue (#22253).