fix(usage): default request recording to basic

[Kayphoon]

Summary

• Default missing usage request record level to Basic in gateway runtime access.
• Make UsageBodyCapturePolicy::default() Basic so fallback policy strips captured request/provider/response bodies.
• Keep explicit Full behavior and existing body size limit parsing unchanged.

Tests

• cargo test -p aether-usage-runtime default_body_capture_policy_strips_body_capture_but_preserves_derived_fields
• timeout 300 cargo test -p aether-gateway usage_runtime_access
• cargo fmt --all -- --check
• timeout 300 cargo check -p aether-usage-runtime -p aether-gateway