.Net: Fix 5796 function calling enum params

[Krzysztof318]

Motivation and Context

Fixes https://github.com/microsoft/semantic-kernel/issues/5796

Description

Type for enum wasn't correctly set in JsonSchemaMapper, it didn't matter for OpenAI but gemini was throwing exception if type isn't specified. Fixed that with string type. Added new unit tests for Gemini and OpenAI. Both passed.

@RogerBarreto @SergeyMenshykh

DataHelper and BertOnyx was updated automatically by formatter.

Contribution Checklist

• [x] The code builds clean without any errors or warnings
• [x] The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• [x] All unit tests pass, and I have added new tests where possible
• [x] I didn't break anyone :smile:

[markwallace-microsoft]

@Krzysztof318 could you take a look at the build error? Thanks.

[Krzysztof318]

@markwallace-microsoft until 19.05 I am out of office