ci: re-add macos-latest build matrix and fix flaky mount timeout

[devin-ai-integration]

Summary

Re-adds the macos-latest job to the build matrix (removed in no macosi) and makes it pass.

The only thing failing on macOS was the VFS Test step: all 17 failures were the same assertion in start_virtual_fs, which waited at most 2s for the FUSE mount to become visible. macFUSE consistently takes just over 2s to mount (CI showed 2.0–2.17s), so every VFS test tripped assert diff < 2, "timed out waiting for vfs to mount". Linux mounts in ~100ms and was unaffected.

Fix is to raise that wait budget:

# start_virtual_fs()
-    assert diff < 2, "timed out waiting for vfs to mount"
+    assert diff < 10, "timed out waiting for vfs to mount"

Workflow change re-introduces the matrix and the macFUSE install step:

runs-on: ${{ matrix.os }}
strategy:
  fail-fast: false
  matrix:
    os: [ubuntu-latest, macos-latest]
...
- name: Install macFUSE
  if: matrix.os == 'macos-latest'
  run: brew install --cask macfuse

fail-fast: false so a failure on one OS doesn't cancel the other.

Link to Devin session: https://app.devin.ai/sessions/5f1b65c493c34e64ab1e1d49ed52f5c1
Requested by: @azuline

[devin-ai-integration]

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