Fix LLM Ops official price source support

[fengren]

Summary

• Limit official provider price source support to backend-registered collectors so unsupported providers are not exposed for automated collection.
• Update LLM Ops official source APIs and tests to use the runtime collector registry.
• Refine the LLM Ops price source modal and batch channel model price preview for auto-collect versus manual maintenance flows.

No linked open Issue found for this follow-up.

Test plan

• .venv/bin/python -m py_compile backend/llm_ops/collection_services.py backend/llm_ops/global_config.py backend/llm_ops/source_collectors/__init__.py backend/llm_ops/source_collectors/official.py backend/llm_ops/source_collectors/registry.py backend/llm_ops/tests/test_ops_bootstrap.py backend/llm_ops/tests/test_source_collectors.py backend/llm_ops/tests/test_views.py
• npx eslint src/components/llm-ops/ChannelModelDrawer.vue src/components/llm-ops/PriceSourceModal.vue --ext .vue --ignore-path ../.gitignore
• git diff --check
• npm run build
• Targeted Django tests not run: local .venv has neither Django nor pytest, and also lacks pip for installing test dependencies.
• Full frontend lint not clean: repository has existing lint errors outside this change; targeted changed Vue files pass.

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