ci(e2e): stabilize preference demo CI execution

[Spherrrical]

Summary

• Switch demo test runner invocation from sh to bash in CI so run_demo_tests.sh executes with the intended shell semantics.
• Add bounded retries for Hurl in llm_routing/preference_based_routing to reduce transient Invalid JSON/EOF flakiness while preserving hard failures.
• Keep other demos unchanged by applying retry logic only to the preference-based routing path.