Fix perf test build against pre-6.0 MDS baselines

[cheenamalhotra]

Summary

The perf pipeline's baseline pass compiles the PerformanceTests project against a released MDS package (-p:ReferenceType=Package -p:MdsPackageVersion=<version>). JsonVsVarcharReadRunner calls SqlDataReader.GetSqlJson, which was introduced in MDS 6.0, so any pre-6.0 baseline broke that pass:

JsonVsVarcharReadRunner.cs(115,32): error CS1061: 'SqlDataReader' does not contain a
definition for 'GetSqlJson' ... [::TargetFramework=net9.0]

Reproduced locally with -p:MdsPackageVersion=5.2.3; 6.0.2 / 7.0.2 / 7.1.0-preview build fine.

Changes

Baseline API gating (Microsoft.Data.SqlClient.PerformanceTests.csproj)

• Derives MdsBaselineMajorVersion from MdsPackageVersion and defines cumulative MDS_GE_6 / MDS_GE_7 constants, so any future benchmark can guard a post-baseline API without new one-off plumbing.
• Tolerates whitespace and a v prefix. Project mode, unpinned Package mode, and unparsable versions all fail toward "current" (sentinel 99999), since those builds reference a current MDS.

JsonVsVarcharReadRunner

• JSON reads are guarded with #if MDS_GE_6, falling back to GetString / GetFieldValueAsync<string>. The async fallback keeps an async read path so the async benchmark stays meaningful.
• Warns loudly from GlobalSetup when the fallback is compiled in — the baseline then measures a different code path, so that benchmark's delta is not comparable.
• Fails with an actionable message when the server lacks native JSON column support, instead of a raw SqlException.
• Validates RowCount and the configured connection string up front.
• Drops tables created by a partially-failed GlobalSetup (BenchmarkDotNet does not guarantee GlobalCleanup runs when setup throws, and table names are unique per run, so leaks accumulate on the perf server).
• GlobalCleanup attempts both drops even if one fails, skips tables never created, aggregates failures, and still clears pools via finally.
• Minor: non-static _rowCount, trailing whitespace.

Docs (eng/pipelines/perf/README.md)

• New "Benchmarks must compile against the oldest baseline" section with the guard pattern.
• Troubleshooting row mapping CS1061 in the baseline pass to the fix.

Validation

Emitted constants:

MdsPackageVersion	constants
5.2.3, v5.2.3, V5.2.3, 0.1.0	(none) → fallback
6.0.2	MDS_GE_6
7.0.2, 7.1.0-preview1.26124.5	MDS_GE_6;MDS_GE_7
latest (unparsable), unpinned, Project mode	all → current

Builds clean in Package mode for 5.2.3 / 6.0.2 / 7.0.2 / 7.1.0-preview1.26124.5 and in Project mode across net8.0/net9.0/net10.0.

• Tests added or updated (perf benchmark project only; no product code changed)
• Public API changes documented (none — no API changes)
• Verified against customer repro (build break reproduced and fixed locally)
• Ensure no breaking changes introduced

No product code is touched, so no release note is needed.

[Copilot]

Pull request overview

This PR fixes the perf pipeline’s baseline build (Package mode with a pinned, released Microsoft.Data.SqlClient version) by ensuring the PerformanceTests project can compile against pre-6.0 baselines that don’t expose newer APIs like SqlDataReader.GetSqlJson. It does this via MSBuild-defined compile-time constants and runner-side API gating, while also improving benchmark setup/cleanup robustness to avoid leaking tables on failure.

Changes:

• Add MSBuild logic in PerformanceTests to derive a baseline major version from MdsPackageVersion and define cumulative MDS_GE_<major> constants (e.g., MDS_GE_6, MDS_GE_7).
• Gate JsonVsVarcharReadRunner’s JSON accessor usage behind #if MDS_GE_6, with meaningful fallbacks and clearer/setup-time warnings when the fallback path is used.
• Document the “oldest baseline must compile” rule and troubleshooting guidance in perf pipeline docs.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/Microsoft.Data.SqlClient/tests/PerformanceTests/Microsoft.Data.SqlClient.PerformanceTests.csproj	Derives baseline major from MdsPackageVersion and defines MDS_GE_6 / MDS_GE_7 compile-time constants to enable API gating.
src/Microsoft.Data.SqlClient/tests/PerformanceTests/BenchmarkRunners/JsonVsVarcharReadRunner.cs	Guards GetSqlJson behind MDS_GE_6 with fallbacks; improves setup validation, failure cleanup, and drop robustness to reduce perf-server table leaks.
eng/pipelines/perf/README.md	Adds guidance for guarding post-baseline APIs with MDS_GE_<major> and links troubleshooting for baseline CS1061 build failures.