test(engine): regression for CR 704.5m aura cleanup when host is exiled

[andriypolanski]

Fixes #5999

Summary

Discord report (#5999): enchantment Auras stay on the battlefield when the permanent they are attached to is exiled.

Investigation showed the engine already handles this correctly on main:

• CR 704.5m — sba::check_unattached_auras puts an Aura into its owner's graveyard when is_valid_attachment_target fails (host not on the battlefield).
• CR 704.4 — SBAs are intentionally deferred while a spell is still resolving. Spells with an optional rider after the exile clause (e.g. Path to Exile's land search) pause on OptionalEffectChoice with the stack already empty; the Aura correctly remains on the battlefield until that rider is answered.
• Once the optional rider resolves, apply_action → run_post_action_pipeline runs the SBA loop and the Aura goes to the graveyard.

The gap was test coverage and a misleading initial repro driver: advance_until_stack_empty() only passes priority and never answers OptionalEffectChoice, so the test stopped with the Aura still on the battlefield even though the engine was waiting for a legitimate player decision.

Changes

• crates/engine/tests/integration/issue_5999_aura_exile_host.rs — four regression tests:
  • Direct exile + SBA — move_to_zone(host, Exile) then check_state_based_actions; Aura graves immediately.
  • Simple exile spell — unconditional Exile target creature. through the full cast/resolve pipeline.
  • Path to Exile — full pipeline with optional land-search rider declined; Aura graves after resolution settles.
  • CR 704.4 pause — asserts Aura stays on the battlefield while the optional rider is pending, then graves once declined.
• crates/engine/tests/integration/main.rs — register the new module.

No engine, parser, or frontend code changes — behavior was already correct.

Test plan

cargo fmt --all -- --check
cargo test -p engine issue_5999

Expected: all four integration tests pass.

cargo test -p engine issue_5999_aura_goes_to_graveyard_when_host_is_exiled_directly
cargo test -p engine issue_5999_aura_goes_to_graveyard_when_host_is_exiled_by_simple_exile_spell
cargo test -p engine issue_5999_aura_goes_to_graveyard_when_enchanted_creature_is_exiled
cargo test -p engine issue_5999_aura_stays_on_battlefield_until_optional_rider_resolves

Related aura SBA regressions (should remain green):

cargo test -p engine issue_2863

Notes for reviewers

• Path to Exile exiles the creature before prompting its controller for the optional land search. During that prompt the Aura is still on the battlefield — that is rules-correct (CR 704.4), not a missing SBA.
• If players report the Aura "stuck" after exile, verify the UI surfaces OptionalEffectChoice when the stack is empty but resolution is not finished (the exiled creature's controller must answer the land-search prompt).
• Spells without an optional post-exile rider (e.g. plain Exile target creature.) graveyard the Aura as soon as the spell fully resolves, with no intermediate pause.

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[matthewevans]

Reviewed — test-only regression coverage is substantively distinct; hold for current required Rust check.

✅ Clean

• issue_5999_aura_exile_host.rs drives the production cast/stack/optional-choice path and proves the CR 704.4 boundary: the Aura remains while Path to Exile is paused, then CR 704.5m removes it after the decision settles. The saw_optional reach guard makes that timing assertion non-vacuous.
• The direct SBA case complements, rather than replaces, existing target-leaves coverage: it pins the Zone::Exile host path, while the two full-pipeline cases cover unconditional and optional-rider resolution.
• Existing sba::check_unattached_auras is the correct ownership seam; this PR adds no production behavior or duplicate mechanism. No parser-surface change means no parse-diff artifact is required.

Recommendation: hold this exact head until the required Rust aggregate finishes, then return it for the authorized handler approval/enqueue pass.

[matthewevans]

Maintainer review complete: the registered production-pipeline tests cover the previously misleading optional-choice boundary and the direct SBA seam; required checks are green.