.Net: Fixed record mapping in Azure CosmosDB MongoDB connector

[dmytrostruk]

Motivation and Context

Closes: #8694

Previous version of record mapping relied on AzureCosmosDBMongoDBNamingConvention class which was applied globally. This approach introduced some issues in unit tests in CI pipeline, like here: https://github.com/microsoft/semantic-kernel/actions/runs/10828226455/job/30043117037?pr=8693, although it's not possible to reproduce it locally. It looks like global naming policy registration sometimes is delayed or ignored, which causes tests to fail occasionally.

To avoid this transient issue, the logic of record mapping was updated to rely only on record property name or BsonElement name and use native BSON {de}serialization logic without global naming policies. Additional logic was added to handle key mapping, since key in MongoDB has a constant name _id, so it should be handled differently comparing to other fields.

Description

Contribution Checklist

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