fix: dropped generation-prompt text in JinjaTemplateChatWrapper

[JoshuaWise]

Description of change

JinjaTemplateChatWrapper was discarding text in the template's generation prompt that comes after the assistant role header. Any static text the template emits between the role header and the model's content -- most importantly a pre-filled reasoning tag like <think> was silently dropped. This PR fixes that behavior, enabling many templates that rely on pre-filling a <think> tag to force the model into reasoning mode, or <think></think> to suppress it.

Fixes #634

Pull-Request Checklist

• Code is up-to-date with the master branch
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• This pull request links relevant issues as Fixes #0000
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[giladgd]

Thanks for the PR!
I noticed it just now after I pushed a fix for the issue you opened (#636)
I'll take a look at it now and see whether your solution is better

[giladgd]

We need to make sure that we don't force-open a thought segment on every render, we should only do it when the model response is empty because we're just starting to generate it.
The rendering also happens after function calls are done (when the results are being put in the context), after context shifts, and some other edge cases, so we shouldn't break those