test: Remove tests that depend on the hosted fake Seam server

The HttpError and Paginator tests ran against the hosted
fake.seam.vc test server, so they fail whenever that service is
unavailable (as it currently is, returning 500s) rather than testing
the SDK. Remove those server-dependent tests and the Fixture helper.

Keep testNonSeamError, which exercises the client's connection-error
handling against an unreachable host and needs no Seam server.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UrUX4ghim2PDnBjXayuLDj

Author: claude