[cDAC] Return no debug info for ReadyToRun images without a DebugInfo section - #135059

Native ReadyToRunInfo::GetDebugInfo (src/coreclr/vm/readytoruninfo.cpp) returns NULL immediately when m_pSectionDebugInfo == NULL. The cDAC mirror, ReadyToRunJitManager.GetDebugInfo, dereferenced ReadyToRunInfo.DebugInfoSection without that check and built a NativeArray from whatever was at address 0.

This now matters because #134690 makes Browser/WASI ReadyToRun publishes pass --strip-debug-info by default, including CoreLib and the framework. iOS, tvOS, and MacCatalyst already default to it. On wasm, linear address 0 is readable, so cDAC decoded garbage. A live run against a stripped browser image threw BadImageFormatException: offset out of bounds (NativeReader.DecodeUnsigned ← NativeArray..ctor ← ReadyToRunJitManager.GetDebugInfo ← DebugInfo_1.GetMethodVarInfo) instead of reporting that there was no debug info. On native targets, the same path fails with a read exception.

Changes

• ReadyToRunJitManager.GetDebugInfo returns TargetPointer.Null (with hasFlagByte = false) when DebugInfoSection is null, matching native.
• docs/design/datacontracts/ExecutionManager.md: the R2R GetDebugInfo description now includes the null-section early return.
• No caller changes were needed. DebugInfo_1.HasDebugInfo, GetMethodNativeMap, GetMethodVarInfo, and GetAsyncSuspensionPoints already treat a null debug-info pointer as "no debug info". The two map/var methods still compute codeOffset.

Tests

New test ExecutionManagerTests.GetDebugInfo_R2R_NoDebugInfoSection_ReturnsNull runs as a [Theory] over StdArchAllVersions (4 arch cases). It builds an R2R module whose DebugInfoSection is null and makes the low 4 KB of the address space readable as zeros, the way wasm linear memory is, so the unfixed code decodes instead of hitting a read fault. It asserts that:

• IExecutionManager.GetDebugInfo returns TargetPointer.Null and hasFlagByte == false
• IDebugInfo.HasDebugInfo is false
• GetMethodVarInfo and GetMethodNativeMap return empty sequences with codeOffset == 4

Results:

• ./build.sh -s tools.cdactests -test: passed. UnitTests 3236/3236, DataGeneratorTests 46/46, UsageTests 4/4 (no stale generated docs).
• ./.dotnet/dotnet test src/native/managed/cdac/tests/UnitTests --filter "FullyQualifiedName~GetDebugInfo_R2R_NoDebugInfoSection": passed, 4/4.
• Mutation check: I removed the null check and reran the same filtered command. It failed 4/4 with System.BadImageFormatException : offset out of bounds at NativeReader.DecodeUnsigned ← NativeArray..ctor ← ReadyToRunJitManager.GetDebugInfo, the same failure as the live wasm run. With the fix restored, it passes 4/4.

This PR has a single concern. It leaves wasm stack-walk and variable-location work to #133890, #135044, and #135055.