Update ConnectorConverter to honer JsonIgnore and JsonPropertyName

[msftshlomiyosef]

Fixes #298
Added GetCachedProperties: Caches PropertyInfo[] per type to avoid repeated reflection.
Added GetPropertyMap: Resolves JSON property names considering:

• JsonPropertyName attributes
• JsonNamingPolicy
• Case insensitivity (optional)
• Results are cached for efficiency

Added support for JsonIgnoreAttribute during serialization to skip ignored properties
Use JsonPropertyNameAttribute to ensure property names are correctly mapped during read/write
Remove manual property name handling in AIEntityConverter

[Copilot]

Pull Request Overview

This PR updates the ConnectorConverter to respect [JsonIgnore] and [JsonPropertyName] attributes, adds reflection-based caching to improve performance, and removes manual property handling in the AIEntityConverter. Corresponding tests have been added to verify derived-entity support, concurrency safety, and duplicate property-name errors.

• Introduce GetCachedProperties and GetPropertyMap with caching in ConnectorConverter
• Honor JsonIgnoreAttribute and JsonPropertyNameAttribute when writing JSON
• Remove handwritten type/context/id mapping in AIEntityConverter and enable attribute-based mapping
• Add tests for derived entities, list serialization, concurrency, and duplicate JSON property detection

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/tests/Microsoft.Agents.Model.Tests/ObjectSerializationTests.cs	Added tests for derived-entity serialization, list-of-activities, concurrency, and duplicate-name errors
src/libraries/Core/Microsoft.Agents.Core/Serialization/Converters/ConnectorConverter.cs	Added property-reflection caching, name-resolution cache, and attribute handling
src/libraries/Core/Microsoft.Agents.Core/Serialization/Converters/AIEntityConverter.cs	Removed manual @type/@context/@id branches in favor of attribute-based mapping
src/libraries/Core/Microsoft.Agents.Core/Models/AIEntity.cs	Uncommented [JsonPropertyName] on AtType, AtContext, and AtId

Comments suppressed due to low confidence (1)

src/tests/Microsoft.Agents.Model.Tests/ObjectSerializationTests.cs:382

• [nitpick] The [ ... ] collection expression may not be recognized in all C# versions or contexts. For clarity and compatibility, consider using new List<Activity> { new DerivedActivity { ... } }.

List<Activity> activities = [new DerivedActivity