test(query): pin SqliteReadOnlySessionEnforcer non-DbException guard

[daniel3303]

Summary

• Adds the negative-path companion to the SQLite read-only detection test: confirms IsReadOnlyViolation requires a DbException before consulting SqlitePrimaryErrorCode.
• Guards against the reflection lookup leaking past the type check — a non-DbException that happens to expose SqlitePrimaryErrorCode == 8 must return false, so a genuine error is never silently neutralized as a read-only violation.

Code changes

• tests/Equibles.AgentQL.UnitTests/Query/ReadOnly/SqliteReadOnlySessionEnforcerTests.cs — new [Fact] feeding a non-DbException that exposes a matching SqlitePrimaryErrorCode and asserting detection returns false. Covers the ex is not DbException guard branch.