fix(helm): drop the CELERY_ACCEPT_CONTENT default

[lewisjared]

Removes the CELERY_ACCEPT_CONTENT default from defaults.env.

The chart set it to the JSON array string ["json", "pickle"]. The v0.16.2 image ignored it, because accept_content was hardcoded in celeryconf/base.py. Climate-REF/climate-ref#850 reads it with env.list(), which splits on commas, so the same value yields ['["json"', '"pickle"]'] and every worker dies at startup with SerializerNotInstalled: No encoder/decoder installed for ["json".

• Drops the default, so the image owns it. The new default is json,ref-json.
• Adds commented examples for CELERY_TASK_COMPRESSION, CELERY_RESULT_COMPRESSION and the comma separated CELERY_ACCEPT_CONTENT, matching the upstream chart.
• Documents the pickle drain path in helm/README.md: set json,ref-json,pickle over a rolling upgrade, then revert.
• Notes that Flower cannot decode ref-json bodies, so its broker queue view will not show task args. Task rows still appear, because worker events are plain JSON.

Safe with either image, so it does not need to land with the version bump. The v0.16.2 image ignores the variable, and the post-#850 image falls back to its own default.

The compose stack still pins v0.16.2, so docker/docker-compose.yaml is untouched. Flower's CELERY_ACCEPT_CONTENT there needs updating in the bump PR.

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📥 Commits

Reviewing files that changed from the base of the PR and between 29b1e04 and df8bcce.

📒 Files selected for processing (3)

• changelog/32.fix.md
• helm/README.md
• helm/values.yaml

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/celery-accept-content-default

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