[vector_graphics_compiler] Use Abi.current() for engine-artifact lookup on Linux ARM64

[HiroyukiTamura]

On Linux ARM64 hosts, vector_graphics_compiler fails with:

Could not locate libpathops at <flutter>/bin/cache/artifacts/engine/linux-x64/libpath_ops.so

Flutter ships the engine binary at .../engine/linux-arm64/libpath_ops.so, but
_initialize_path_ops_io.dart and _initialize_tessellator_io.dart hardcoded
'linux-x64' on Linux and never looked at the arm64 directory.

Both loaders now route through a new pure engineArtifactSubpath(...) helper
that maps Abi.current() to the engine-artifact subpath. The helper is
unit-tested across every supported Abi, the linux-arm64 fix, and the
unsupported-host branch. Behavior on windows-x64, darwin, and linux-x64 hosts
is unchanged. Bumps the package to 1.2.6.

Fixes flutter/flutter#158865.

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[gemini-code-assist]

Code Review

This pull request adds support for linux-arm64 hosts in the vector graphics compiler. It dynamically determines the platform directory for pathops and the tessellator loader using Abi.current() instead of hardcoding linux-x64. Additionally, the package version is bumped to 1.2.4 in pubspec.yaml and documented in CHANGELOG.md. There are no review comments to address.

[stuartmorgan-g]

[x] I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under.

Thanks for the contribution! Where is this item reflected in the PR?

[HiroyukiTamura]

@stuartmorgan-g
Hi, sorry I had to write test exemption. I updated PR description.
It require extracting platform mapping logic for the testing, but I felt like too much for this one line changing.

[stuartmorgan-g]

It require extracting platform mapping logic for the testing, but I felt like too much for this one line changing.

Why specifically is extracting logic to make a fix testable too much? Is there something make it infeasible?

[gaaclarke]

lgtm, thanks

[gaaclarke]

@HiroyukiTamura you're going to have to rebase please

[stuartmorgan-g]

We can resolve the changelog/pubspec conflicts as part of landing (it's trivial in the web UI, as long as the branch isn't locked).

But before we can do that the testing question needs to be resolved.

[gaaclarke]

But before we can do that the testing question needs to be resolved.

That's probably not @HiroyukiTamura's responsibility though, right? We'd need linux arm bots which I know is something the infra team is investigating for flutter. I'm not sure what the situation is for packages.

[stuartmorgan-g]

Marking as change requested to reflect my comments above.

[stuartmorgan-g]

We'd need linux arm bots

We would need Linux arm bots for an end-to-end test, but not for a unit test. See my comment above where I responded to the PR author's argument that refactoring to make this unit testable was "too much" for this change.

[HiroyukiTamura]

I pushed fixes.

• I extracted the platform mapper logic and added unit tests.
  Also I narrowed the unsupported branch: ABIs that Flutter doesn't ships engine artifacts for (linuxIA32/Arm/Riscv*, windowsIA32).
• The unsupported-host print was using Platform.localeName (which prints "en_US" etc.). I switched it to Abi.current()