test: add offline providers.json validation (closes #2)

[Jah-yee]

Summary

Adds an offline validation test suite () that checks without calling any provider APIs.

What is validated

Check	Details
Unique IDs	No duplicate provider ids
Required fields	id, name, description, defaultModel are non-empty
HTTP(S) baseUrl	Valid URL with http:// or https:// protocol
No embedded credentials	No user/password in URL; no apiKey/token/secret in auth object
Supported protocol	One of: openai-chat, gemini, anthropic
Supported auth type	One of: bearer, x-api-key, api-key, x-goog-api-key, google-api-key, none, custom

Acceptance criteria (from issue #2)

• ✅ Unique IDs
• ✅ Supported protocols and auth types
• ✅ Valid HTTP(S) base URLs
• ✅ Required names/descriptions
• ✅ Absence of embedded credentials
• ✅ Does not call provider APIs

Test result

# tests 12
# pass 12
# fail 0

Run with: node --test tests/providers-validate.test.mjs

[Jah-yee]

Friendly ping — checking in on the offline providers.json validation tests. Happy to address any feedback.