[.NET SDK] Upgrades OpenAPI Generator from 7.12.0 to 7.26.0

[gbiernacki-dropbox]

Upgrades OpenAPI Generator from 7.12.0 to 7.26.0 for the .NET SDK and adds a native .NET 10 build. This will ship as 2.6.0.

Breaking changes

None. Package validation against the published 2.5.0 package reports no compatibility issues.

Changes visible to customers (backwards-compatible)

• .NET 10 build: the package now contains net8.0 and net10.0 builds (#22562, #22234).
• User-Agent: the default User-Agent is no longer percent-encoded (OpenAPI-Generator/2.6.0/csharp) (#24923).
• Async: internal awaits use ConfigureAwait(false), avoiding deadlocks in synchronous contexts (#21244).
• New helper: ClientUtils.ParameterToJsonString() for JSON-encoded header parameters (#25083).

Internal changes (not visible to customers)

• RetryConfiguration.RetryPolicy / AsyncRetryPolicy keep their Policy<RestResponse> / AsyncPolicy<RestResponse> types; upstream changed them to interfaces (#20821).
• Optional parameters use default instead of default(T) (same compiled values) (#21147).
• Server URL variable check fixed (#21122).
• Templates refreshed from 7.26.0; generation runs on the .NET 10 SDK image, and SDK CI installs .NET 8 and 10.

.NET 8 and .NET 10

• What changed: net10.0 is added next to net8.0. It is built only when the .NET 10 SDK is used, so the SDK still builds from source with the .NET 8 SDK (net8.0 only).
• Backwards compatibility:
  • .NET 8 and .NET 9 apps keep using the net8.0 build from the same NuGet package, unchanged.
  • A packaging check fails dotnet pack without the .NET 10 SDK, so a release can never ship without the net10.0 build.
  • Unit tests run on both .NET 8 and .NET 10.