Add integration coverage for route_request dispatch behavior

[prk-Jr]

Description

route_request in crates/fastly/src/main.rs owns request dispatch for static assets, admin endpoints, auction paths, and publisher fallback behavior, but it currently lacks direct integration coverage.

Done when

• Add integration tests that exercise the main route branches through the Fastly entry path.
• Verify admin routes reject unauthenticated requests.
• Verify unknown routes fall back to publisher proxy behavior.
• Keep this issue scoped to route dispatch coverage rather than full frontend/container setup.

Affected area

• Fastly runtime

Additional context

Source: test_gap.md, section Route Request Logic (Integration Test Required).
Parent issue: #396
Related: #398, #400