.Net: Update AgentDefinition to match the latest schema changes

[markwallace-microsoft]

Motivation and Context

Update AgentDefinition to match the latest schema changes from the declarative agent work group

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 😄

PR Overview

This pull request updates the AgentDefinition to align with the latest schema changes defined by the declarative agent work group. Key changes include:

• Replacing the Metadata property type in AgentDefinition from IDictionary to a new AgentMetadata type.
• Introducing and updating YAML type converters (AgentMetadataTypeConverter) to correctly deserialize agent metadata.
• Updating agent type identifiers from "azureai_agent" to "foundry_agent" across relevant files and tests.

Reviewed Changes

File	Description
dotnet/src/Agents/UnitTests/Yaml/AgentDefinitionYamlTests.cs	Adds new tests to validate YAML deserialization using the updated AgentMetadata.
dotnet/src/Agents/Abstractions/Definition/AgentMetadata.cs	Introduces a new type to capture agent metadata properties with extension data support.
dotnet/src/Agents/Yaml/AgentMetadataTypeConverter.cs	Provides custom YAML deserialization for AgentMetadata with updated property handling.
dotnet/src/Agents/AzureAI/Definition/AzureAIAgentFactory.cs	Updates agent type constant to "foundry_agent" to reflect schema changes.
dotnet/src/Agents/UnitTests/Yaml/AzureAIKernelAgentYamlTests.cs	Adjusts YAML content in tests to use the new "foundry_agent" value.
dotnet/src/Agents/Yaml/AgentDefinitionYaml.cs	Adds the AgentMetadataTypeConverter to the YAML deserialization pipeline.
dotnet/src/Agents/Abstractions/Definition/AgentDefinition.cs	Changes the Metadata property type to use AgentMetadata instead of a generic dictionary.
dotnet/src/Agents/UnitTests/Yaml/KernelAgentYamlTests.cs	Updates YAML tests to align with the new agent type identifier.

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