refactor(gateway): replace flat-file import shims with installed package

Install bot_bottle via pip in Dockerfile.gateway instead of COPYing
individual .py files flat under /app/. This eliminates the try/except
import shims in egress_addon_core, dlp_detectors, egress_addon,
supervise, supervise_server, and git_http_backend that existed only
to support the flat-bundle layout.

Adds bot_bottle/constants.py as a single source of truth for
IDENTITY_HEADER and GIT_GATE_TIMEOUT_SECS, removing the duplicated
literal definitions in egress_addon.py, supervise_server.py,
git_http_backend.py, and git_gate_render.py.

Test files updated to match: test_supervise_server.py drops the
sys.path.insert hack in favour of direct package imports; the
egress_addon test shims no longer pre-populate sys.modules with a
bare egress_addon_core alias.

tests/unit/test_egress_addon_request_flow.py

@@ -190,9 +190,6 @@ def _ensure_shims() -> None:
         setattr(mh, "Response", _Response)
     if not hasattr(mh, "HTTPFlow"):
         setattr(mh, "HTTPFlow", object)
-    if "egress_addon_core" not in sys.modules:
-        import bot_bottle.egress_addon_core as _core
-        sys.modules["egress_addon_core"] = _core
 
 
 _ensure_shims()