Add local proxy integration-test fixtures

[francomano]

Description

Add reproducible integration tests for the network adapters using local proxy fixtures.

Codebase starting point

• core/adapters/direct.go
• core/adapters/http_proxy.go
• core/adapters/socks.go
• core/adapters/common.go
• core/checks/route_trace/check_test.go as an example of package-local tests

Scope

• Local HTTP proxy fixture.
• Local HTTPS proxy fixture.
• Local SOCKS4 fixture.
• Local SOCKS5 fixture.
• Authenticated proxy cases.
• Failure cases: bad credentials, timeout, refused CONNECT.

Acceptance Criteria

• go test ./... can run fixtures locally without external services.
• Adapters are tested for HTTP requests, CONNECT/TLS and port checks.
• Tests are stable in CI.