feat: embed workflow in generated PNG metadata

[TimPietruskyRunPod]

Summary

Closes #139.

Generated PNGs currently cannot be dragged back into ComfyUI to recover the source workflow — because the worker was suppressing PNG metadata in two places:

1. `src/start.sh` launched ComfyUI with `--disable-metadata`
2. `handler.py` did not forward `extra_pnginfo.workflow` in the `/prompt` payload

`extra_pnginfo.workflow` is what the `Save Image` node reads to populate the `workflow` key embedded in PNG metadata (separate from the API-format `prompt`). Without both, the round-trip from output PNG → editor workflow doesn't work.

Bonus fix: crystools NoneType crash

This PR also fixes a separate bug discovered while investigating #167. Without `extra_pnginfo` in the payload, ComfyUI-Crystools' `Save image with extra metadata` node crashes with:

```
AttributeError: 'NoneType' object has no attribute 'copy'
```

at `nodes/image.py:392` (`extra_pnginfo_new = extra_pnginfo.copy()`) when `with_workflow=True`. Reproduced on a fresh pod today; PR fixes it.

Test plan

• Reproduced the crystools NoneType crash on `main` (5.8.5 image)
• After applying this PR's patch on a live pod, crystools `Save image with extra metadata` runs successfully with `with_workflow=True` and the handler returns a normal-sized image response
• Validate end-to-end on a serverless endpoint built from this PR (via ci: build base image on every pull request #223 once merged) that the resulting PNG actually contains both `prompt` and `workflow` keys in its metadata

Trade-off

Each generated PNG is now ~10–50 KB larger (workflow JSON embedded). For users who want smaller responses, they can route through `PreviewImage` instead of `Save Image`, or strip metadata client-side. Worth documenting in a follow-up.

[TimPietruskyRunPod]

Validation status update

Test	Status	Notes
Diff logic in isolation	✅	Verified the new `extra_data` initialization + `api_key_comfy_org` merge logic correctly produces both `extra_pnginfo.workflow` and `api_key_comfy_org` keys when applicable, and only `extra_pnginfo` when no key
--disable-metadata removal from `start.sh`	✅	Manually inspected, two occurrences (SERVE_API_LOCALLY=true branch and default branch) both removed
crystools NoneType crash fix	✅	Reproduced the crash on main's handler (5.8.5-flux1-dev-fp8 pod), applied this PR's diff to /handler.py in the running pod, restarted, re-submitted the workflow — completed successfully (585 KB response, no crash)
End-to-end PNG metadata embedding on a serverless endpoint	⚠️ pending	Built image runpod/worker-comfyui:fix-enable-png-workflow-metadata-base published, deployed to a fresh serverless endpoint with a LoadImage → SaveImage workflow. Worker came up healthy, picked up the job, then went unhealthy mid-execution. Could not retrieve worker logs (RunPod serverless doesn't expose them via API). The same image pattern worked when I tested via the pod manually, so likely an infra-level issue on that specific assigned host (same class as #186's original CUDA mismatch). Re-test once #223 merges and a real release goes out

The code change is correct and the crystools side-effect is verified. The final PNG-metadata roundtrip test (decode the returned PNG with PIL and confirm workflow chunk is present) hasn't been validated on a serverless endpoint yet — the test pod test wasn't a useful substitute because the pod image had --disable-metadata baked into its start.sh (only the handler.py was hot-patched).

I'd recommend merging this anyway and validating on the next release — the surface area is small, the diff logic checks out, and the worst case is "PNG metadata not embedded" which is the current state of main.

[TimPietruskyRunPod]

Verified both changes are in the built image:

```
$ docker run --rm --platform linux/amd64 --entrypoint cat \
runpod/worker-comfyui:fix-enable-png-workflow-metadata-base /start.sh | grep main.py
python -u /comfyui/main.py --disable-auto-launch --listen --verbose "${COMFY_LOG_LEVEL}" --log-stdout &
python -u /comfyui/main.py --disable-auto-launch --verbose "${COMFY_LOG_LEVEL}" --log-stdout &
```

Both `python /comfyui/main.py` invocations are missing `--disable-metadata` ✅. And:

```
$ docker run --rm --platform linux/amd64 --entrypoint bash \
runpod/worker-comfyui:fix-enable-png-workflow-metadata-base \
-c "grep 'extra_pnginfo' /handler.py | head -2"
# extra_pnginfo.workflow is what ComfyUI embeds into generated PNGs so users
"extra_data": {"extra_pnginfo": {"workflow": workflow}},
```

The `extra_pnginfo` payload change is in handler.py ✅. So both halves of the fix are correctly baked. End-to-end PNG embedding test (decode returned PNG and confirm `workflow` chunk) still pending a live deploy that doesn't hit the host-side infra issue I ran into earlier today.