feat(http): support custom fetch transport

[h1065153539-create]

Summary

• Add a fetch option to GuildPassClientConfig for runtime-specific transports, tracing, proxies, and tests.
• Pass the custom transport into HttpClient while keeping existing retry-only and hook-only constructor usage working.
• Validate missing fetch support clearly and document the integration pattern.

Tests

• npm run test:run -- tests/httpClient.test.ts tests/client.test.ts
• npm run typecheck
• npm run build
• npm run test:run
• npm run test:smoke
• npm run lint (passes with existing warnings)
• git diff --check

Closes #55

[Lakes41]

Very well done