Allow custom fetch transport injection

[Lakes41]

Summary

Allow consumers to provide a custom fetch implementation so the SDK works reliably across Node.js, browsers, Edge runtimes, tests, and specialised network environments.

Current Behaviour

HttpClient calls the global fetch directly. This makes it harder to test without global stubbing and limits consumers who need custom agents, tracing, proxy support, or runtime-specific fetch implementations.

Expected Behaviour

The SDK should support dependency injection for the HTTP transport while preserving the current default behaviour.

Suggested Implementation

Add a fetch?: typeof fetch or lightweight transport option to GuildPassClientConfig. Pass it into HttpClient, defaulting to globalThis.fetch. Validate that a fetch-compatible function exists at runtime and throw a clear config error if it does not.

Files or Areas Likely Affected

• src/client/GuildPassClient.ts
• src/http/httpClient.ts
• src/http/http.types.ts
• src/config/sdkConfig.ts
• src/errors/errorCodes.ts
• tests/client.test.ts
• tests/httpClient.test.ts
• docs/integration-guide.md

Acceptance Criteria

• GuildPassClient accepts a custom fetch-compatible function
• HttpClient uses the injected transport instead of global fetch
• Existing usage without a custom transport still works
• Missing fetch support produces a clear GuildPassError
• Tests no longer require global fetch stubbing for all HTTP cases
• Documentation includes a short custom transport example

Additional Notes

This should not introduce Node-only transport dependencies because the SDK targets browser and Edge runtimes.

[shadrach68]

Hi maintainer, I am confident that I can resolve this issue before 24hrs if assigned to me because I have the technical expertise to resolve it.