Silent state transition failures due to incorrect MongoDB UpdateOne error handling

[jrwbabylonlab]

The transitionState function in internal/db/delegation.go:59-81 incorrectly assumes MongoDB's UpdateOne() returns mongo.ErrNoDocuments when no document matches the filter. In reality,
UpdateOne() returns success with MatchedCount == 0. The code discards the result and only checks for errors, causing state transitions to report success when no update occurred.

Root Cause:

_, err := client.UpdateOne(ctx, filter, update)  // Result discarded
if err != nil {
    if errors.Is(err, mongo.ErrNoDocuments) {    // Dead code - never true for UpdateOne
        return &NotFoundError{...}
    }
}
return nil  // Returns success even when MatchedCount == 0

Affected Functions:

• TransitionToUnbondingState
• TransitionToUnbondedState
• TransitionToWithdrawnState

Impact:

• State transitions silently fail when delegation doesn't exist or is in wrong state
• processExpiredDelegations deletes expire check records even when transitions fail, leaving delegations stuck
• Unbonding tx data not saved on silent failure of TransitionToUnbondingState

Fix:

Check result.MatchedCount == 0 and return NotFoundError:
result, err := client.UpdateOne(ctx, filter, update)
if err != nil {
    return err
}
if result.MatchedCount == 0 {
    return &NotFoundError{
        Key:     stakingTxHashHex,
        Message: "Delegation not found or not in eligible state to transition",
    }
}
return nil