Ash Framework: Filter authorization misapplies impossible bypass/runtime policies

Summary

When using filter authorization, two edge cases could cause the policy compiler/authorizer to generate a permissive filter:

1. Bypass policies whose condition can never pass at runtime were compiled as
   OR(AND(condition, compiled_policies), NOT(condition)).
   If the condition could never be true at runtime, the NOT(condition) branch evaluated truthy and the overall expression became permissive.

2. Runtime policy scenarios that reduce to “no checks are applicable” (an empty SAT scenario) were treated as an empty clause and dropped instead of being treated as false, which could again produce an overly broad (permissive) filter.

These bugs could allow reads to return records that should have been excluded by policy.

Impact

Projects that rely on filter-based authorization and define:

• bypass ... do ... end blocks whose condition(s) are only resolvable at runtime and can never pass in a given request context, or
• runtime checks that simplify to an empty scenario for a clause

may unintentionally generate a permissive query filter, potentially returning unauthorized data.

Actions primarily affected: reads guarded by filter policies. Non-filter (e.g., hard forbid) policies are not impacted.

Technical details

This patch corrects two behaviors:

• Ash.Policy.Policy.compile_policy_expression/1 now treats bypass blocks as
  AND(condition_expression, compiled_policies)
  instead of OR(AND(...), NOT(condition_expression)). This removes the permissive NOT(condition) escape hatch when a bypass condition never passes.

• Ash.Policy.Authorizer now treats empty SAT scenarios (scenario == %{}) as false, ensuring impossible scenarios do not collapse into a no-op and inadvertently widen the filter. The reducer also normalizes nil → false consistently when building auto_filter fragments.

Relevant changes are in:

• lib/ash/policy/policy.ex (bypass compilation)
• lib/ash/policy/authorizer/authorizer.ex (scenario handling / auto_filter normalization)
• Tests added: test/policy/filter_condition_test.exs (RuntimeFalsyCheck, RuntimeBypassResource) validate the corrected behavior.

Workarounds

• Avoid bypass policies whose conditions are only decidable at runtime and may be perpetually false in some contexts; prefer explicit authorize_if/forbid_if blocks without bypass for those cases.
• Add an explicit final forbid_if always() guard for sensitive reads as a belt-and-suspenders fallback until user can upgrade.
• Where feasible, replace runtime-unknown checks with strict/compile-time checks or restructure to avoid empty SAT scenarios.

How to tell if user is affected

User is likely affected if ALL of the following are true:

• Uses filter authorization; and
• Defines bypass block with access_type :runtime without any policies after it; or
• Defines bypass blocks whose conditions are evaluated at runtime (e.g., checks with strict_check/3 returning :unknown and a runtime check/4 that may never succeed in some contexts) without any policies after it

A quick sanity test is to issue a read expected to return no rows under such a bypass or runtime-falsy condition and verify it indeed returns []. The included test bypass works with filter policies demonstrates the corrected, non-permissive behavior.