test: tighten regression guard and hoist httpx import

Per review:
- Hoist `import httpx` out of the two test function bodies and into
  the module imports at the top of
  test_astrolabe_chunk_context.py.
- Simplify the regression guard in
  test_management_chunk_context_endpoint.py to use
  `mock.assert_awaited_once_with(...)` instead of manually unpacking
  call_args. This is stricter — it fails loudly on signature change —
  and matches the canonical pattern for asserting mock calls.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/integration/test_astrolabe_chunk_context.py

@@ -25,6 +25,7 @@ import logging
 import re
 import uuid
 
+import httpx
 import pytest
 
 from tests.conftest import create_mcp_client_session
@@ -208,8 +209,6 @@ async def test_chunk_context_endpoint_uses_app_password(
 @pytest.mark.timeout(60)
 async def test_chunk_context_endpoint_requires_authentication():
     """Direct HTTP hit at /api/v1/chunk-context without a bearer must 401."""
-    import httpx
-
     async with httpx.AsyncClient() as client:
         response = await client.get(
             "http://localhost:8003/api/v1/chunk-context",
@@ -235,8 +234,6 @@ async def test_chunk_context_endpoint_rejects_invalid_bearer():
     `validate_token_and_get_user`, which must turn into a clean 401/404 from
     the HTTP layer, not an opaque 500.
     """
-    import httpx
-
     async with httpx.AsyncClient() as client:
         response = await client.get(
             "http://localhost:8003/api/v1/chunk-context",