test(egress): cover egress_addon adapter; drop coverage omit

The mitmproxy adapter `egress_addon.py` was omitted from coverage
because it can't import on the host (mitmproxy is sidecar-only) and
only its log-redaction helpers were exercised. Add a request/response
flow suite that stubs mitmproxy and drives the adapter glue:
introspection, allowlist enforcement, auth strip+inject, git
push/fetch blocking, the outbound-DLP block/redact/supervise policy
branches (including the operator approval round-trip), inbound
response scanning, and WebSocket frame scanning.

Removes the `bot_bottle/egress_addon.py` omit from `.coveragerc`;
the adapter now reports ~76% covered.

Closes #286

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NkwFXLFff9PYPy4wgVBJp9

.coveragerc

@@ -4,6 +4,5 @@ source = .
 
 [report]
 omit =
-    bot_bottle/egress_addon.py
     bot_bottle/cli/tui.py
     tests/*