Fix provider enum type casting for zod validation

[Copilot]

Addressed type safety issue where Object.values(Providers) returns string[] but z.enum() requires [string, ...string[]].

Changes

• Extract Object.values(Providers) into typed constant with explicit cast to [string, ...string[]]
• Update schema to use typed constant instead of inline Object.values() call

Implementation

// Before
provider: z.enum(Object.values(Providers)).default(DEFAULT_PROVIDER).optional()

// After  
const providerValues = Object.values(Providers) as [string, ...string[]];
provider: z.enum(providerValues).default(DEFAULT_PROVIDER).optional()

Maintains existing Providers enum usage while satisfying zod's tuple type requirements.

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