[Metal] Fall back to the system default device (#24839)

On an M5 Max running macOS 26.6 inside a Codex sandbox, we observed
`MTLCopyAllDevices()` return an empty array in a fresh process even
though `MTLCreateSystemDefaultDevice()` returned a valid device. IREE
snapshots the first enumeration result when creating its Metal driver,
so an initially empty result leaves that driver permanently without
devices.

Instead, when enumeration is empty, we use the default device as a
one-device fallback.

Fixes #24834.

AI assistance disclosure: I used OpenAI Codex substantially to inspect
the implementation, trace the failure, implement the change, and prepare
this PR (but not write this description). I reviewed and understand the
resulting code and analysis.

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Signed-off-by: Jeremy Howard <github@jhoward.fastmail.fm>