fix(dotnet): tolerate numeric ping timestamps

[xianjianlf2]

Fixes #2091.

Summary

• apply the existing DateTimeOffset timestamp converter to PingResponse.Timestamp
• extend that converter to accept ISO-8601 strings, numeric epoch milliseconds, and stringified epoch milliseconds
• apply the same converter to generated Rpc.PingResult.Timestamp through the C# generator and current generated output
• add serialization tests for both public and typed RPC ping result shapes

Root cause

PingResponse.Timestamp and generated Rpc.PingResult.Timestamp used the default DateTimeOffset JSON converter, which expects a string and rejects older numeric epoch-millisecond payloads from some CLI builds. Since ping is used during startup protocol verification, that deserialization error prevents the .NET client from starting.

Validation

• Not run: dotnet test test/GitHub.Copilot.SDK.Test.csproj --filter SerializationTests could not run because dotnet is not installed in this environment (zsh:1: command not found: dotnet).