test: add coverage for decision constants and NoopStore

[Pranjal0410]

pkg/decision/decision_test.go (3 tests)

• Wire protocol constants have unique values
• Byte values match expected encoding (Allow=0x00, Sanitise=0x01, Block=0x02)
• All constants fit single byte (wire protocol constraint)

internal/state/noop_test.go (5 tests)

• Get returns nil for any key
• Get returns nil across different session IDs
• Set accepts any input without panic
• Set-then-Get still returns nil (stateless guarantee)
• Multiple Set calls maintain no state

All 57 sidecar tests passing. No code modified — tests only.