Add logging for OpenAPI schema default value type mismatches

[Copilot]

Problem

The current ApplyDefaultValue method in OpenAPI schema generation fails when there are type mismatches between default values and their target types, particularly in F# scenarios. For example:

[<DefaultParameterValue(10)>] // int32 default
member this.GetData(id: uint64) = // uint64 parameter

This causes InvalidCastException during JsonSerializer.SerializeToNode() calls, preventing OpenAPI schema generation from completing successfully.

Solution

This PR implements a logging-based approach that:

1. Attempts normal JSON serialization first - Maintains backward compatibility for all existing working scenarios
2. Logs warnings on serialization exceptions - When type mismatches occur, logs a descriptive warning message using ILogger
3. Omits default values for type mismatches - No default value is applied to the schema when types don't match, as recommended
4. Remains trim-safe - No reflection-based operations are added

Implementation

The change adds logging infrastructure and modifies the ApplyDefaultValue method in JsonNodeSchemaExtensions.cs:

try
{
    schema[OpenApiSchemaKeywords.DefaultKeyword] = JsonSerializer.SerializeToNode(defaultValue, jsonTypeInfo);
}
catch (Exception ex) when (ex is InvalidCastException or NotSupportedException or InvalidOperationException)
{
    // Log warning when there's a type mismatch that prevents serialization
    logger?.DefaultValueTypeMismatch(defaultValue.GetType().Name, jsonTypeInfo.Type.Name);
    // Do not apply the default value when there's a type mismatch
}

Testing

Added comprehensive tests that verify:

• Warning logs are emitted for type mismatches ([DefaultValue(10)] ulong id)
• No default values are applied when types don't match
• Existing scenarios continue to work unchanged

Compatibility

• ✅ Backward compatible - Existing working scenarios are unchanged
• ✅ Trim-safe - No new reflection or dynamic code generation
• ✅ Handles F# scenarios - Resolves [<DefaultParameterValue>] type mismatches with proper logging
• ✅ Follows recommendations - Logs warnings and omits mismatched default values instead of applying incorrect values

Fixes scenarios where OpenAPI generation would previously fail with InvalidCastException, now producing valid schemas while alerting developers to type mismatches through logging.

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[dotnet-policy-service]

Greetings human! You've submitted a PR that modifies code that is shared with https://github.com/dotnet/runtime . Please make sure you synchronize this code with the changes in that repo!

[captainsafia]

@copilot It's recommended that the type of the default value match the type defined in the schema so this option is not viable. Update the PR to:

• Handle serialization exceptions and log a warning to the user
• Add new tests that verify that logs are emitted for cases where the default value type and the parameter type don't match

[Copilot]

@copilot It's recommended that the type of the default value match the type defined in the schema so this option is not viable. Update the PR to:

• Handle serialization exceptions and log a warning...

Updated the implementation to log warnings instead of using string fallback for default value type mismatches. The solution now:

1. Handles serialization exceptions and logs warnings using ILogger
2. Added tests that verify warning logs are emitted for type mismatches
3. No longer applies default values when types don't match

Changes implemented in commit 4e363d3.