.Net: Fix: KernelJsonSchemaBuilder now excludes read-only properties from generated schema

[sajidkamaljoy]

Motivation

Currently, KernelJsonSchemaBuilder includes read-only properties (properties with no setter) in the generated JSON schema. This causes the AI to attempt to generate values for fields it cannot write to (e.g., computed properties), which wastes tokens and can lead to hallucinated data that is silently ignored.

Changes

• Modified KernelJsonSchemaBuilder.GetDefaultOptions to include a JsonTypeInfo modifier.
• The modifier (ExcludeReadOnlyProperties) inspects the JsonTypeInfo and removes any property where Set is null.
• Added a regression test in KernelJsonSchemaTests to verify that read-only properties are absent from the generated schema while writable properties remain.

Validation

• Added new unit test ItShouldExcludeReadOnlyPropertiesFromSchema.
• Ran all tests in SemanticKernel.UnitTests.Functions.

[sajidkamaljoy]

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