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

• Typecheck passes
• Invalid config values ("api": "anthropic") caught with clear error message
• Valid configs load normally
• 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

🤖 Generated with Claude Code

[rekram1-node]

if you want this to get merged it'd be nice if it was cleaned up, this should be a very very simple change like maybe 3 lines max

[brettheap]

Updated PR to address feedback:

• ✅ Simplified to 1 focused commit (down from 37)
• ✅ Only the minimal runtime validation (~9 lines)
• ✅ Rebased on latest upstream/dev
• ✅ Removed config-time validation (Phase 2) - saved for future PR
• ✅ Removed all documentation/openspec files

The fix now validates baseURL format before passing to the SDK, preventing ERR_INVALID_URL when users misconfigure provider settings. Just as you requested - "a very very simple change" 👍