[cDAC] Fix stale Continuation MT bug

[rcj1]

_continuationMethodTablePointer and _continuationSingletonEEClassPointer are not filled on startup, but on the first creation of a Continuation without metadata. Therefore, on a live target we may read a null MT if we only read upon contract creation. Update this such that if these are still null, we try again to read them.

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[Copilot]

Pull request overview

This PR makes continuation-related globals in the cDAC RuntimeTypeSystem contract resilient to late initialization by lazily re-reading the globals when cached values are still null, rather than only capturing them at contract construction time.

Changes:

• Make continuation MT / singleton EEClass pointers lazily refreshed in RuntimeTypeSystem_1 when still null.
• Add the same lazy refresh behavior in TypeValidation so MethodTable validation can succeed once the target initializes the globals.

Show a summary per file

File	Description
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/RuntimeTypeSystemHelpers/TypeValidation.cs	Lazily re-reads continuation globals when cached values are null, enabling validation paths to recover on live targets.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/RuntimeTypeSystem_1.cs	Lazily re-reads continuation globals in the main contract implementation so IsContinuationWithoutMetadata can work after late initialization.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 1