Provider presets and custom entries can leave haiku, sonnet, or opus blank when the main model is the intended fallback. Normalize those mappings at both the Settings UI and ProviderService boundaries so persisted provider config and generated runtime env stay consistent.
Constraint: Users may configure only a main model for custom providers.
Rejected: Keep blank secondary model slots | runtime env would still receive empty ANTHROPIC_DEFAULT_* values.
Confidence: high
Scope-risk: narrow
Directive: Keep UI and ProviderService normalization aligned for provider model mapping changes.
Tested: Not run per maintainer request.
Not-tested: Local test suite skipped by request.
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