[cDAC][wasm] Register platform GC-info decoding - #135055

… 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.

> [!NOTE]
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Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>