fix: enforce strict JSON numbers

[PANiXiDA]

What changed

• Configured AddHttp to use JsonNumberHandling.Strict for HTTP JSON serialization.
• Added regression coverage for strict numeric deserialization, string enum compatibility, and numeric OpenAPI schemas.
• Documented the JSON contract in the README.

Why

ASP.NET Core web defaults allow numbers to be read from JSON strings. In .NET 10, OpenAPI 3.1 reflects that setting as integer | string and number | string, which prevents clients such as Kiota from generating scalar numeric properties.

How to verify

dotnet restore
dotnet format
dotnet build --configuration Release
dotnet test --configuration Release

All 75 package tests pass. The equivalent setting was also applied temporarily to the Tactical Heroes API host and all 487 solution tests passed.

Compatibility

• Breaking behavior: quoted numbers in JSON request bodies are rejected; callers must send JSON numeric tokens.
• OpenAPI numeric schemas change from unions to integer or number.
• String properties and enums configured for string serialization are unchanged.
• No migrations or configuration changes are required.