Add default value support for all primitive types in elicitation schemas (SEP-1034)

[Copilot]

SEP-1034: Support default values for all primitive types in elicitation schemas

This PR implements SEP-1034 to add default value support for all primitive types in elicitation schemas.

Changes Made

• Add Default property to StringSchema class
• Add Default property to NumberSchema class
• Add Default property to EnumSchema class
• Update the Converter.Read method to deserialize default values for all types using a switch statement
• Update the Converter.Write method to serialize default values for all types
• Use literal true/false values for boolean defaults in converter
• Use is not null pattern consistently for all nullable checks
• Add comprehensive tests for string default values
• Add comprehensive tests for number default values
• Add comprehensive tests for enum default values
• Build and test the changes - all tests passing (20 tests pass, 1 skipped)
• Run code review - no issues found
• Run security checks - no vulnerabilities found

Summary

All primitive schema types (StringSchema, NumberSchema, EnumSchema, BooleanSchema) now support default values as specified in SEP-1034. The converter correctly handles serialization and deserialization of these default values based on the JSON token type using a switch statement with literal boolean values.

Default values can be set programmatically when creating schemas, and will be properly serialized/deserialized through the JSON converter.

Example

var schema = new ElicitRequestParams
{
    Message = "Configure email",
    RequestedSchema = new()
    {
        Properties = new Dictionary<string, ElicitRequestParams.PrimitiveSchemaDefinition>
        {
            ["recipients"] = new StringSchema { Default = "team@example.com" },
            ["priority"] = new NumberSchema { Default = 1 },
            ["status"] = new EnumSchema { Enum = ["draft", "sent"], Default = "draft" }
        }
    }
};

Future: Automatic Default Value Extraction

A test has been added (currently skipped) that demonstrates automatic extraction of default values from optional parameters in positional records when using ElicitAsync<T>(). This will work once AIJsonUtilities is updated to support extracting default values from optional parameters. The test uses a positional record FormWithDefaults with optional parameters like string Name = "John Doe" and verifies that defaults are included in the generated elicitation schema.

• Fixes SEP-1034 : Support default values for all primitive types in elicitation schemas #908

Original prompt

---

This section details on the original issue you should resolve

<issue_title>SEP-1034 : Support default values for all primitive types in elicitation schemas</issue_title>
<issue_description>Is your feature request related to a problem? Please describe.

Add support for SEP-1034, which is accepted for the next version of the MCP spec.

Describe the solution you'd like

All features of SEP-1034 are supported as described.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.
</issue_description>

<agent_instructions>Per the changes in the MCP specification, update all schema types contained in ElicitRequestParams.cs to include a Default property (similar to how BooleanSchema already does this).

Additionally update the converter in the same file so that default keywords are mapped correctly to the new properties.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes SEP-1034 : Support default values for all primitive types in elicitation schemas #908

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[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• http://crl3.digicert.com:443/DigiCertCSRSA4096RootG5.crl
  • Triggering command: dotnet test tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj /p:TargetFrameworks=net9.0 --filter FullyQualifiedName~Elicit_Typed_With_Defaults_Maps_To_Schema_Defaults (http block)
• http://crl3.digicert.com:443/NETFoundationProjectsCodeSigningCA2.crl
  • Triggering command: dotnet test tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj /p:TargetFrameworks=net9.0 --filter FullyQualifiedName~Elicit_Typed_With_Defaults_Maps_To_Schema_Defaults (http block)
• http://crl3.digicert.com:443/sha2-assured-cs-g1.crl
  • Triggering command: dotnet test tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj /p:TargetFrameworks=net9.0 --filter FullyQualifiedName~Elicit_Typed_With_Defaults_Maps_To_Schema_Defaults (http block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
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