Add early-exit validation to MapAsync for already mapped buffers and …

[rajv79]

Summary of Commit

This PR adds an early-exit check in BufferBase::APIMapAsync() to prevent calling
MapAsync on a buffer that is already mapped (either via MapAsync or mappedAtCreation).

It improves developer feedback and avoids redundant validation, while returning
a null future and triggering the error callback early.

Key Changes

• Checks the current buffer state before ValidateMapAsync()
• Emits a warning using dawn::EmitWarning
• Returns a null Future on invalid use
• Invokes the callback with WGPUMapAsyncStatus_Error

Test Coverage

Added MapAsyncFailsWhenAlreadyMapped in BufferValidationTests to verify behavior.

CI Note (Temporary Workaround)

This PR includes a temporary local patch to work around a known Protobuf build issue on macOS
It can be removed once the CI or Protobuf dependency is updated.

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[rajv79]

Hi @nico @precision @dj2 ,
I tested the change locally, and it worked. However, the CI is failing with a build error related to Protobuf and constinit, which I wasn't expecting. After looking into it, I found it’s a known issue on macOS with Xcode/Clang.

To help CI proceed, I temporarily disabled DAWN_ENABLE_INSTALL in the cache config file. It doesn’t affect the feature I added — just helps bypass the CMake export error.

Please let me know if there’s a better way to handle this. Thanks so much!