Migrate union roundtrip tests to C# 15 native unions

[sadicangel]

The Docker union tests currently exercise interface/object representations, and Chr skips mixed-object unions. Migrate both suites to C# 15 native unions while keeping interface coverage in the fast serializer tests and C# 14 unit snapshots.

• Enable union preview generation in both roundtrip projects and register the generated Notification and PaymentRecord builders for Chr.
• Replace the old union roundtrip tests with native record and mixed-union cases. Verify the decoded runtime case type as well as its value.
• Extend the payment fixture with fixed and enum branches while retaining null, string, integer, and record coverage. Make shared test serialization helpers work with both representations.
• Keep the Apache enum case explicitly skipped pending apache/avro#4031. Chr now runs every mixed-union case.
• Simplify README prerequisites to state the SDK 10.0.100 minimum directly.

Validation:

• Release solution build: zero warnings/errors.
• Three unit suites and both fast integration suites: 1,196 passed.
• Docker roundtrips: Apache 15 passed / 2 skipped; Chr 16 passed / 1 skipped. Skips cover the existing self-reference limitation and the Apache enum issue.
• Packed consumers: 6 runs on SDK 10.0.401 and 9 on SDK 11.0.100-preview.6.26359.118 passed; generated-source parity passed.
• git diff --check passed.

The minimum-SDK command, ./tests/AvroSourceGenerator.PackageCompatibility/Run-Compatibility.ps1 -PackagePath artifacts/packages/AvroSourceGenerator.0.0.0-native-roundtrip-review.nupkg -SdkVersion 10.0.100 -ExpectedRoslynApiVersion 5.0 -OutputDirectory artifacts/manifests/native-roundtrip-review/minimum, could not run because SDK 10.0.100 is not installed locally. CI retains that baseline check.