Add DacMode-based SOS test legs to runtime-diagnostics pipeline

[max-charlamb]

Replaces the useCdac/noFallback booleans in the SOS test legs with a single dacMode parameter and adds a 4th leg. The shared windows_x64 build now uploads the freshly built cDAC (mscordaccore_universal) as a separate artifact for the cdac leg to consume.

SOS legs (windows_x64 Release, testInterpreter):

• cDAC -- dacMode cdac -- standalone cDAC via SOS (downloads + overlays the built cDAC next to sos.dll)
• cDAC_fallback -- dacMode cdacfallback -- in-box DAC hosts cDAC, with fallback
• cDAC_verify -- dacMode cdacverify -- in-box DAC hosts cDAC, no fallback (verifies)
• DAC -- dacMode dac -- legacy DAC only

Depends on the DacMode support added in dotnet/diagnostics#5901. To exercise it before that merges, queue runtime-diagnostics with diagnosticsBranch pointing at the diagnostics PR's merge ref.

Sample run: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1481626&view=logs&j=d359ac0d-403b-5f4f-9a78-93671d33ea9d

[dotnet-policy-service]

Tagging subscribers to this area: @steveisok, @tommcdon, @dotnet/dotnet-diag
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

Updates the runtime-diagnostics Azure DevOps pipeline SOS test legs to use a single dacMode selector (instead of useCdac/noFallback), adds an additional SOS leg, and wires up publishing/downloading the freshly-built standalone cDAC so the cdac leg can exercise the runtime-under-test’s own mscordaccore_universal.

Changes:

• Replace useCdac/noFallback booleans with a single dacMode string parameter through the SOS leg templates.
• Add a 4th SOS leg (cDAC_verify) and adjust existing leg naming/modes (cDAC_fallback, DAC).
• Publish the built mscordaccore_universal output as a separate artifact and download it only for the dacMode: cdac leg.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
eng/pipelines/runtime-diagnostics.yml	Adds the 4th SOS leg, switches legs to dacMode, and uploads the built cDAC as a dedicated artifact.
eng/pipelines/diagnostics/sos-test-leg.yml	Replaces boolean toggles with dacMode and conditionally downloads the cDAC artifact for the cdac mode.
eng/pipelines/diagnostics/runtime-diag-job.yml	Maps dacMode to the corresponding -dacMode / -cdacPath arguments passed to the diagnostics build/test harness.

[hoyosjs]

/azp run runtime-diagnostics

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[max-charlamb]

/ba-g cDAC pipeline Chang to sync with diagnostics