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.

bot_bottle/supervise.py

@@ -37,40 +37,22 @@ from abc import ABC
 from dataclasses import dataclass
 from pathlib import Path
 
-try:
-    from .supervise_types import (
-        ACTION_OPERATOR_EDIT,
-        AuditEntry,
-        Proposal,
-        Response,
-        STATUSES,
-        STATUS_APPROVED,
-        STATUS_MODIFIED,
-        STATUS_REJECTED,
-        TOOLS,
-        TOOL_EGRESS_ALLOW,
-        TOOL_EGRESS_BLOCK,
-        TOOL_EGRESS_TOKEN_ALLOW,
-        TOOL_GITLEAKS_ALLOW,
-        TOOL_LIST_EGRESS_ROUTES,
-    )
-except ImportError:
-    from supervise_types import (  # type: ignore[import-not-found,no-redef]  # pylint: disable=import-error,no-name-in-module
-        ACTION_OPERATOR_EDIT,
-        AuditEntry,
-        Proposal,
-        Response,
-        STATUSES,
-        STATUS_APPROVED,
-        STATUS_MODIFIED,
-        STATUS_REJECTED,
-        TOOLS,
-        TOOL_EGRESS_ALLOW,
-        TOOL_EGRESS_BLOCK,
-        TOOL_EGRESS_TOKEN_ALLOW,
-        TOOL_GITLEAKS_ALLOW,
-        TOOL_LIST_EGRESS_ROUTES,
-    )
+from .supervise_types import (
+    ACTION_OPERATOR_EDIT,
+    AuditEntry,
+    Proposal,
+    Response,
+    STATUSES,
+    STATUS_APPROVED,
+    STATUS_MODIFIED,
+    STATUS_REJECTED,
+    TOOLS,
+    TOOL_EGRESS_ALLOW,
+    TOOL_EGRESS_BLOCK,
+    TOOL_EGRESS_TOKEN_ALLOW,
+    TOOL_GITLEAKS_ALLOW,
+    TOOL_LIST_EGRESS_ROUTES,
+)
 
 
 try: