Support image input modalities for custom providers

[importantitemzl-tech]

Summary

• preserve declared image input modalities for custom provider models in Pi and OpenCode
• expose modalities in the Admin custom-provider editor without dropping model pricing
• normalize legacy and malformed persisted modality values safely during provider hydration

Verification

• custom provider, Pi image request, and OpenCode tests
• Admin model-field round-trip tests
• TypeScript checks and ESLint
• Prettier and diff checks

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