[DAC] fix dac_gc_heap marshalling

[max-charlamb]

Found while working on #119324

This API was giving different data from the cDAC, I manually inspected the data structure in WinDBG and found that the DAC was giving incorrect data.
It looks like this is the only API to marshal a dac_gc_heap without LoadGcHeapData. I think it was reading the class as if it were laid out sequentially in memory as opposed to using the offsets in g_gcDacGlobals->gc_heap_field_offsets.

I modified the API to use LoadGcHeapData in the same way as other APIs.

Adding test to the diagnostics repo dotnet/diagnostics#5562

diagnostics pipeline with failures on main: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1140784&view=results
runtime-diagnostics leg running with new test: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1140803&view=results

[Copilot]

Pull Request Overview

This PR fixes an issue in the DAC (Data Access Component) where the ServerGCInterestingInfoData function was incorrectly marshalling dac_gc_heap data by reading memory sequentially instead of using proper field offsets.

• Replaces direct pointer access with the LoadGcHeapData helper function
• Ensures correct data marshalling by using field offsets from g_gcDacGlobals->gc_heap_field_offsets
• Brings consistency with other DAC APIs that marshal dac_gc_heap structures

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[max-charlamb]

/backport to release/10.0

[github-actions]

Started backporting to release/10.0: https://github.com/dotnet/runtime/actions/runs/17564021296

[github-actions]

@max-charlamb an error occurred while backporting to "release/10.0", please check the run log for details!

Error: @max-charlamb is not a repo collaborator, backporting is not allowed. If you're a collaborator please make sure your dotnet team membership visibility is set to Public on https://github.com/orgs/dotnet/people?query=max-charlamb

[max-charlamb]

/backport to release/10.0

[github-actions]

Started backporting to release/10.0: https://github.com/dotnet/runtime/actions/runs/17564098954