Fix InvalidCastException for nullable enum parameter default values i…

[AnsElgyarDev]

Fixes #69864

Description

ParameterInfo.DefaultValue returns the underlying integral value (a boxed Int32) for Nullable<TEnum> parameters rather than a TEnum. ApplyParameterInfo passed that value to ApplyDefaultValue, which serialized it using the JsonTypeInfo for TEnum?, causing an InvalidCastException during OpenAPI document generation.

This converts the default value with Enum.ToObject before applying it, matching what ParameterDefaultValue.TryGetDefaultValue and RequestDelegateFactory already do for nullable enums.

Tests

• Added Status? = Status.Pending, TaskStatus? = TaskStatus.Canceled and Status? = null cases to RouteParametersWithDefaultValues.
• Added a test for a [FromBody] nullable enum parameter with a default value.
• Without the fix, the first two query cases and the body test fail with the InvalidCastException from the issue. With the fix, all Microsoft.AspNetCore.OpenApi.Tests pass.

MVC actions go through the same ApplyParameterInfo code path, but I did not add a dedicated MVC test.

[dotnet-policy-service]

Thanks for your PR, @AnsElgyarDev. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[AnsElgyarDev]

The only failing check is "Local development validation / Test: Windows local development validation / Build (No NodeJS)". The log shows RZ1021 errors in src/Components/test/testassets/Components.TestServer Razor files, which are unrelated to this change (it only touches Microsoft.AspNetCore.OpenApi and its tests). All Microsoft.AspNetCore.OpenApi.Tests pass locally. Could a maintainer re-run it if needed? Thanks!