Player owes bottom cards before making another mulligan decision

[pascalberger]

When playing against AI I sometimes see the following error:

[ERROR] processQueue error (local): Engine error: Invalid action: Player PlayerId(1) owes bottom cards before making another mulligan decision

[mike-theDude]

Triage analysis

Clarified summary
During games against the AI, the engine intermittently rejects a mulligan decision with an error stating the player still owes bottom cards before making another mulligan decision.

Investigation findings
The error is raised when a mulligan decision arrives for a player whose mulligan entry is still in its bottom-cards phase. Two AI-side paths can produce exactly that:

1. The AI's fallback action selector reads the first entry in the pending mulligan set rather than the entry belonging to the player it is acting for. When more than one player is still resolving a mulligan, it can therefore emit a declare-phase decision while its own entry is owed bottoms.
2. That same fallback answers a bottom-cards phase with an empty card selection. Bottom-selection validation requires exactly the owed count, so an empty submission is rejected whenever the count is non-zero, leaving the obligation unmet and the fallback looping.

The main AI mulligan policy resolves the acting player's own entry correctly, so this is specific to the fallback path taken after repeated decision failures.

Reproduction status
Confirmed as an actionable defect from code inspection. A saved game state from a game where the error appears would let the fix be pinned to one of the two paths.

Likely scope
area:ai for the fallback action selector; area:engine for the mulligan phase contract it violates.

Acceptance criteria

• The AI fallback resolves the pending mulligan entry for the player it is acting for, never the first entry in the set.
• A bottom-cards phase is answered with exactly the owed number of cards.
• A game against the AI completes mulligans without this error, including when both players mulligan.

Related issues
#6342 reports the same obligation going unmet from the human side, where an order-sensitive legality check rejects the submission. Related root symptom, distinct cause; tracked separately.

[pascalberger]

This is still happening after #6628 has landed

[matthewevans]

Linked a new Discord report with the same mulligan-bottom-card error; the reporter also lost the game after refresh.