fix(benchmarks): make SetupTeardownTests JSON tests Native AOT compatible

[thomhurst]

Problem

The README benchmark table shows — for TUnit (AOT) on the Setup/teardown lifecycle row. The AOT benchmark run for SetupTeardownTests crashes:

• JsonOperationTest / AsyncJsonOperationTest serialize an anonymous type with System.Text.Json.JsonSerializer.Serialize(data).
• PublishAot=true sets JsonSerializerIsReflectionEnabledByDefault=false, so reflection-based STJ throws InvalidOperationException at runtime.
• 2 failing tests → non-zero exit → CliWrap ExecuteBufferedAsync throws in RuntimeBenchmarks.TUnit_AOT → BenchmarkDotNet records NA → README renders —.

This is a benchmark-suite bug, not a TUnit hooks bug — [Before(Test)]/[After(Test)] work fine under AOT.

Fix

Replace the anonymous type with a concrete JsonPayload record serialized via a source-generated JsonSerializerContext. Works under Native AOT and compiles for all four framework variants (shared file).

Verification

No MSVC linker on this machine, so full AOT link wasn''t possible locally. Instead, reproduced the exact AOT failure mode by building the TUnit variant with -p:JsonSerializerIsReflectionEnabledByDefault=false (the same feature switch PublishAot flips) and running --treenode-filter "/*/*/SetupTeardownTests/*":

• Confirmed "System.Text.Json.JsonSerializer.IsReflectionEnabledByDefault": false in runtimeconfig.json
• 22/22 tests pass

Also compile-checked XUNIT3, NUNIT, MSTEST variants — all build clean.

After merge, the next weekly Speed Comparison run should fill in the missing AOT cell.

[codacy-production]

Not up to standards ⛔

🔴 Issues 1 minor

Alerts:
⚠ 1 issue (≤ 0 issues of at least minor severity)

Results:
1 new issue

Category	Results
CodeStyle	1 minor

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[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

Summary: This PR replaces an anonymous-type JSON payload in tools/speed-comparison/UnifiedTests/SetupTeardownTests.cs with a concrete JsonPayload record serialized via a source-generated JsonSerializerContext, fixing a Native AOT crash (reflection-based System.Text.Json is disabled under PublishAot). Verified the fix is correctly scoped (benchmark tooling, not core engine — CLAUDE.md's Dual-Mode/Snapshot Testing rules don't apply here), directly satisfies the AOT Compatible rule, compiles cleanly, and preserves the original serialization semantics (same field names/values, no behavioral regression).