[cDAC] Restore Frame-walk fallback in Thread.GetContext

[Copilot]

The E_NOTIMPL Frame-walk fallback removed in 5391e9e is still needed for data targets that don't implement GetThreadContext. The native side is being restored in a separate PR; this change brings the equivalent path to cDAC.

Changes

• Thread_1.GetContext: When TryGetThreadContext returns false, walk the thread's Frame chain (modeled after IStackWalk.GetFrames) instead of throwing InvalidOperationException.

  • For an InterpreterFrame, fill the context from the top InterpMethodContextFrame via the existing UpdateContextFromCurrentFrame (which calls SetContextToInterpMethodContextFrame).
  • For other frames, update a cleared context from the frame and accept the first one with both SP and PC non-zero (e.g. RedirectedThreadFrame, InlinedCallFrame, DynamicHelperFrame), setting RawContextFlags = FullContextFlags to mark SP/PC/FP as valid. This mirrors the native DT_CONTEXT_CONTROL [| DT_CONTEXT_INTEGER] intent without per-architecture branching.
  • If no frame yields a usable context, return a zeroed context (thread is not running managed code).

• docs/design/datacontracts/Thread.md: Updated the GetContext pseudocode to document the new fallback.

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[Copilot]

Pull request overview

This PR changes the cDAC IThread.GetContext implementation to fall back to deriving a thread context from the thread’s explicit Frame chain when the data target can’t provide an OS thread context, and updates the Thread data contract design doc accordingly.

Changes:

• Update Thread_1.GetContext to return the OS context when TryGetThreadContext succeeds, otherwise derive a context by walking the thread’s Frame chain.
• Add a helper (GetContextFromFrames) that selects an InterpreterFrame context (via InterpMethodContextFrame) or the first frame producing non-zero SP/IP.
• Update docs/design/datacontracts/Thread.md pseudocode to document the fallback behavior.

Note: I did not run build/tests as part of this review.

Show a summary per file

File	Description
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/Thread_1.cs	Adds frame-walk fallback path for GetContext when TryGetThreadContext doesn’t provide a context.
docs/design/datacontracts/Thread.md	Documents the updated GetContext behavior and fallback selection rules.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 3

[Copilot]

Copilot's findings

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