Improve string format handling for gemini

[dmontagu]

Addresses an issue demonstrated in #582.

In particular, this PR pops the 'format' field from string schemas (the presence of which is causing API errors), and uses it to modify the field description, which seems to result in responses that don't have errors and parse successfully.

This PR doesn't go as far as was requested in that issue, but the approach I've used here seems reasonable given we do get API errors without it, it seems like a common usage scenario, and I also confirmed manually with gemini-1.5-flash that the changes in this PR result in responses that parse successfully, at least for the specific prompt/model I was using (basically the one from that issue).

Weirdly, the VertexAI docs say that:

For the format field, Vertex AI supports the following values: date, date-time, duration, and time. The description and format of each value is described in the OpenAPI Initiative Registry

But in practice it didn't seem to work with any of these. If/when those format values do start to work, we could revert this change.

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

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0.0% Coverage on New Code
0.0% Duplication on New Code

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