[Invoker] Unconditional unreserve_slot calls inflate quota for vqueue invocations

[tillrohrmann]

Bug Description

Several handlers in crates/invoker-impl/src/lib.rs call self.quota.unreserve_slot() unconditionally, but vqueue invocations never call reserve_slot (they use the scheduler's permit system via a non-empty Permit). This causes the legacy quota's available_slots counter to drift upward over time when vqueue invocations terminate through these paths.

Correct Pattern

handle_invocation_task_closed (line 1082) and the yield branch in handle_invocation_task_should_yield (line 1284) correctly check ism._permit.is_empty() before unreserving:

if ism._permit.is_empty() {
    self.quota.unreserve_slot();
}

Affected Handlers (Missing Guard)

Handler	Line	Path
handle_invocation_task_suspended	1119	Invocation suspends (v1)
handle_invocation_task_suspended_v2	1182	Invocation suspends (v2)
handle_abort_invocation	1347	Single invocation abort
handle_abort_partition	1458	Partition-wide abort
handle_error_event / OnTaskError::Pause	1606	Error → pause
handle_error_event / OnTaskError::Kill	1670	Error → kill

Impact

• For Quota::Unlimited (the default): unreserve_slot is a no-op, so there is no impact in the default configuration.
• For Quota::Limited with vqueues enabled simultaneously: available_slots drifts upward beyond the configured limit, effectively defeating the concurrency limit.

Fix

Gate all unreserve_slot calls with if ism._permit.is_empty(), matching the pattern in handle_invocation_task_closed.

Found During

Code review of #4354 (invoker memory budget). This bug is pre-existing and unrelated to the memory budget changes (except for the yield handler which was already fixed).