`import gateway` reaches agent/conversation_compression.py through
gateway/session.py -> agent/turn_context.py, and a module-level
`from agent.auxiliary_client import AuxiliaryExplicitCancellation`
there dragged in agent.credential_pool -> hermes_cli.auth -> httpx at
import time. Minimal consumers that import the gateway package with only
the lightweight wire deps (websockets/aiohttp/pyyaml/requests) — e.g. the
gateway-gateway cross-repo live E2E suite — now crash with
ModuleNotFoundError: No module named 'httpx' before running anything.
Move the import to call time inside compress_context(), matching the
existing lazy-import pattern for aux_progress_hook /
aux_interrupt_protection in the same function. Both uses of the exception
class (the raise and the except) are inside compress_context's dynamic
extent, so behavior is unchanged.
Add tests/gateway/test_gateway_import_hygiene.py: a fresh-interpreter
probe that blocks httpx/openai/anthropic on the meta path and imports
gateway.relay.ws_transport, so an eager heavyweight import on the
gateway path fails CI instead of breaking downstream repos.
Verified: probe fails on current main, passes with this change; also
reproduced in a real minimal venv (pip install websockets aiohttp pyyaml
requests) where `from gateway.relay.ws_transport import
WebSocketRelayTransport` now succeeds with httpx absent.
