WebGPU EP crashes on macOS when running concurrent multi threaded inference

### Describe the issue

**Note:** Re opening this issue as a follow up to #27592, which was automatically closed by the stale bot despite being an active (and I can't click to re open), reproducible bug. Confirmed still present on latest ORT (`v1.28`) so wanted to create a new anyway.

The WebGPU Execution Provider crashes on macOS (Apple Silicon) when multiple threads create sessions and run inference concurrently. The crash is non-deterministic and manifests as different errors on each run, indicating a race condition in Dawn's Metal backend.

Observed crashes (non-deterministic, varies per run)
Run 1 — Metal assertion:
```
**-[AGXG15GFamilyCommandBuffer blitCommandEncoderCommon:]:832:**
failed assertion `A command encoder is already encoding to this command buffer'
```

**Run 2 — Memory corruption:**
` malloc: *** error for object 0x784797e90: pointer being freed was not allocated
`

**Run 3 — Trace trap:**
` [1] 56040 trace trap cargo run --release -- version-RFB-640.onnx`

Notes:
- Single-threaded session creation and inference works fine - Sequential (non-concurrent) multi-session usage works fine
- The same workload works correctly with the CoreML EP

### To reproduce

minimal Repo: https://github.com/altunenes/ort-webgpu-thread-crash

git clone https://github.com/altunenes/ort-webgpu-thread-crash.git
cargo run --release -- version-RFB-640.onnx
(simple face detection model)

The repro creates 2 persistent WebGPU sessions, then spawns 5 threads that each create a session and run inference concurrently while the main thread also runs inference on the persistent sessions.

### Urgency

_No response_

### Platform

Mac

### OS Version

26.2

### ONNX Runtime Installation

Built from Source

### ONNX Runtime Version or Commit ID

1.28

### ONNX Runtime API

Other / Unknown

### Architecture

X64

### Execution Provider

Other / Unknown

### Execution Provider Library Version

_No response_

Start with the linked minimal repository and run `cargo run --release version-RFB-640.onnx` on macOS to reproduce the concurrent WebGPU session crash. Compare the concurrent workload with single-threaded and sequential multi-session runs, then verify that concurrent inference completes without the reported Metal assertions, memory errors, or trace traps.

Written by the indexing model from the issue text.