refactor: introduce IHttpClient seam for testable auth

[Taure]

Introduces an IHttpClient seam so AsobiAuth can be unit-tested without a live backend, and adds the first network-free auth tests.

Why

AsobiAuth called a concrete HttpClient (UnityWebRequest) with no injection point, so the only coverage of auth request shape + token-storage side effects was the live-backend PlayMode SmokeTest. There was no way to assert "login stores the returned tokens" or "an error status does not store tokens" in CI without a server.

What

• New internal interface IHttpClient declaring the exact public surface HttpClient already exposes (Get/Post/PostJson/Put/PutJson/GetRaw/PutRaw/Delete + AccessToken). HttpClient now implements it.
• AsobiClient.Http is typed IHttpClient; a new internal AsobiClient(AsobiConfig, IHttpClient) constructor lets tests inject a fake. The public constructor delegates to it with the real HttpClient, so nothing changes for consumers.
• The 16 Asobi* API classes are untouched - they already call methods that live on the interface.
• Tests/Runtime/AsobiAuthTests.cs (EditMode): a FakeHttpClient records the last path/body and returns canned responses. Covers login credential shape + token storage + token propagation to the HTTP layer, register display-name defaulting, OAuth token storage, and that a 401 aborts without storing tokens.

Notes

• These are Unity EditMode tests (they use PlayerPrefs via AsobiClient), so CI's EditMode runner executes them with no backend - unlike the PlayMode smoke. They are not headless-dotnet runnable because the runtime depends on UnityEngine.
• InternalsVisibleTo("Asobi.Tests") was already present, so the fake sees the internal interface.
• Independent of the guest-auth PR (feat: add guest auth (device create-or-resume + upgrade) #21); both follow the identical auth pattern and compose cleanly.

Local validation

Unity/IL2CPP can't build headless in this environment; verified by careful review that the interface matches HttpClient's signatures exactly and that no .Http. member used anywhere falls outside it. CI validates the full build + EditMode run.