Fixed two critical data loss bugs where user-added custom endpoints were discarded:
1. **AddProviderDialog**: Form submission ignored values.meta from ProviderForm and
re-inferred URLs only from presets/config, causing loss of endpoints added via
speed test modal. Now prioritizes form-collected meta and uses fallback inference
only when custom_endpoints is missing.
2. **ProviderForm**: Edit mode always returned initialData.meta, discarding any
changes made in the speed test modal. Now uses mergeProviderMeta to properly
merge customEndpointsMap with existing meta fields.
Changes:
- Extract mergeProviderMeta utility to handle meta field merging logic
- Preserve other meta fields (e.g., usage_script) during endpoint updates
- Unify new/edit code paths to use consistent meta handling
- Add comprehensive unit tests for meta merging scenarios
- Add integration tests for AddProviderDialog submission flow
Impact:
- Third-party and custom providers can now reliably manage multiple endpoints
- Edit operations correctly reflect user modifications
- No data loss for existing meta fields like usage_script
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