Strengthen Egress Gate mutation aggregation tests

[johnnygreco]

Summary

• prove that final request mutations are equivalent to applying three gates sequentially
• cover progressive and overlapping regex redaction across multiple gates
• cover interacting header operations, empty body replacement, and terminal allow behavior
• add exact mutation-limit, local request-limit, protected-header, and extension-capability coverage
• verify a three-gate redaction result through the gRPC boundary

Why

Egress Gate applies each gate to an updated immutable request and returns one
aggregate mutation result to OpenShell. These tests make that equivalence
explicit and cover the observable boundary cases that could change the final
forwarded request.

This is a test-only change.

Validation

• make check — 248 tests passed
• make check-py311 — 248 tests passed
• formatting, lint, type checks, and dependency audit passed in both validation runs