Implement full JsonNumberHandling support

[Copilot]

Previously only AllowReadingFromString was partially supported. This adds complete support for all JsonNumberHandling flags per the Microsoft documentation.

Changes

FieldConverter.cs

• Added WriteAsString support: numbers serialize as JSON strings when flag is set
• Added AllowNamedFloatingPointLiterals support: handles "NaN", "Infinity", "-Infinity" for float/double types
• Added validation: throws JsonException when named literals are used without the flag

JsonNumberHandlingTests.cs

• Added 6 tests covering all flag combinations and error cases

Example Usage

// Serialize numbers as strings
var options = new JsonSerializerOptions();
options.NumberHandling = JsonNumberHandling.WriteAsString;
// Output: {"value": "123.45"}

// Read named floating-point literals
options.NumberHandling = JsonNumberHandling.AllowReadingFromString | 
                         JsonNumberHandling.AllowNamedFloatingPointLiterals;
// Accepts: {"value": "NaN"}, {"value": "Infinity"}

// Flags work in combination
options.NumberHandling = JsonNumberHandling.WriteAsString | 
                         JsonNumberHandling.AllowReadingFromString;
// Full round-trip support for numbers as strings

All numeric types (int32, int64, float, double, etc.) are supported. Named literals only apply to float/double as per spec.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Use NumberHandling</issue_title> <issue_description>https://learn.microsoft.com/en-us/dotnet/api/system.text.json.serialization.jsonnumberhandling </issue_description>

<agent_instructions>Implement properly JsonNumberHandling. It was already implemented for AllowReadingFromString but we need full support. </agent_instructions>

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

• Fixes Havret/Protobuf.System.Text.Json#65

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