Remove cDAC fallback testing mode

[noahfalk]

Once dotnet/diagnostics#5972 merges, cDac verify mode should be a strictly stronger testing mode that includes all the same test coverage. No need to run both verify and fallback modes separately.

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[Copilot]

Pull request overview

This PR removes the separate “cDAC fallback” testing mode by making fallback behavior strictly allowlist-based in the legacy shim and by deleting the dedicated cDAC_fallback SOS pipeline leg, leaving cDAC_verify as the stronger coverage mode.

Changes:

• Removed the CDAC_NO_FALLBACK environment-variable toggle and made fallback delegation depend only on an explicit allowlist.
• Removed the cDAC_fallback leg from the runtime diagnostics pipeline (and updated related pipeline commentary).
• Kept cDAC_verify as the remaining “compare against legacy” SOS mode (dacMode: cdacverify).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Legacy/LegacyFallbackHelper.cs	Removes the env-var switch and enforces allowlist-only fallback decisions (with stderr logging).
eng/pipelines/runtime-diagnostics.yml	Drops the cDAC_fallback SOS test leg and updates the pipeline overview/comments accordingly.
eng/pipelines/diagnostics/sos-test-leg.yml	Updates header comments to reflect removal of the fallback leg.

Suppressed comments (1)

src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Legacy/LegacyFallbackHelper.cs:38

• CanFallback() returns true for allowlisted methods or allowlisted caller files, but the doc comment only mentions methods. Updating this keeps the docs aligned with the actual allowlist logic.

 /// <summary>
/// Returns <c>true</c> if the calling method is allowed to delegate to the legacy DAC.
/// Returns <c>true</c> only for allowlisted methods.
/// All fallback attempts (allowed and blocked) are logged to stderr.
/// </summary>