fix(samples): isolate Razor compilation in CI

[ReubenBond]

The documentation workflow intermittently corrupts Razor-generated sources in its long-lived compiler server, producing spurious RZ1021 and cascading C# errors in the ShoppingCart sample. The identical commit passed after the job restarted, matching the compiler-server corruption documented in dotnet/roslyn#85656 and dotnet/roslyn#85196.

DOTNET_TieredPGO=0 is not sufficient evidence of resolution because dotnet/roslyn#85656 includes later recurrence reports with that setting active. This change instead adds an opt-in Build-Samples.ps1 switch which passes UseSharedCompilation=false only to the sample solution build. That bypasses the stateful compiler server for the affected Razor workload while preserving the standard sample compilation requirement. The CI job uses no-incremental builds and previously completed in under five minutes against a 45-minute timeout, leaving ample capacity for the isolated compiler processes.

The runtime fix in dotnet/runtime#132631 is scheduled for 10.0.13. Once global.json selects an SDK carrying runtime 10.0.13 or later, remove the switch and validate shared compilation across repeated pull-request and scheduled Documentation runs. If the Razor corruption recurs, restore the bounded switch and continue investigation with generated sources and binlogs against dotnet/roslyn#85656.

Mitigates #11235

[Copilot]

Copilot review overview

🟢 Approval recommended

The change is a scoped, low-risk CI mitigation applied only to the affected workflow job and is clearly documented for future removal.

Review effort: Lite
Findings: None

What changed in this PR

Mitigates intermittent Razor compilation failures in the documentation/samples CI job by disabling Tiered PGO via an environment variable, aligning with the documented workaround for the suspected runtime/JIT issue until the repo’s pinned SDK/runtime is updated.

Changes:

• Adds DOTNET_TieredPGO=0 at the job level for the “Build documentation projects and samples” workflow job.
• Documents the linked tracking issue and removal criteria in the workflow comments.

File	Description
.github/​workflows/​docs.yml	Sets a job-scoped environment variable to disable Tiered PGO to stabilize Razor compilation in CI for docs/samples.

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[github-actions]

Code coverage

Metric	Pull request	Current main	Variance
Lines	82.06% (112,189 / 136,709)	82.12% (112,272 / 136,709)	-0.0607 pp
Branches	71.24% (32,270 / 45,295)	71.32% (32,306 / 45,295)	-0.0795 pp

Report-only conclusion: regressed.

The current-main baseline is commit 2e40fa8a3b and uses the same reviewed coverage matrix.

Coverage combines every CI test matrix job, including providers, CodeGen, .NET 8/10, Linux, Windows, and macOS, using canonical physical source and branch identities.

The comparison remains report-only while normal line and branch variance is calibrated.

Coverage details

• Pull request commit: 5b4f492825
• View the pull request test run and coverage artifacts
• View the trusted coverage reporting run

[Copilot]

Copilot review overview

🟢 Approval recommended

The change is narrowly scoped to CI/sample build behavior, is opt-in at the script level, and cleanly adds diagnostics without affecting default local builds.

Review effort: Lite
Findings: None