fix(provider): validate baseURL format to prevent ERR_INVALID_URL

[brettheap]

Summary

• Adds URL format validation for provider baseURL/api fields
• Prevents cryptic ERR_INVALID_URL errors when invalid values like "anthropic" or "" reach the SDK
• Defense-in-depth with two validation layers

Changes

Phase 1: Runtime Validation (provider.ts)

• Validates baseURL is a proper URL (http:// or https://) before passing to SDK
• Invalid values are filtered out, letting SDK use its default endpoint

Phase 2: Config-Time Validation (config.ts)

• Catches invalid api field values at config load time
• Hybrid strategy: validates against models.dev cache when available, falls back to URL format check
• Provides helpful error messages with hints

Root Cause

The @ai-sdk/* SDKs use nullish coalescing (??) for baseURL defaults, which doesn't catch empty strings or non-URL values—only null/undefined.

Test Plan

• [x] Typecheck passes
• [x] Invalid config values ("api": "anthropic") caught with clear error message
• [x] Valid configs load normally
• [x] SDK uses default URL when invalid baseURL is filtered

Example Error Message

Invalid provider configuration:

  provider.anthropic.api: Invalid value "anthropic"
    Hint: Remove the "api" field to use the default anthropic endpoint

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