feat: support stateful CometUDFs

[mbutrovich]

Which issue does this PR close?

Closes #.

Rationale for this change

Peeled off from #4267 to keep that PR scoped to codegen. The cache-shape change is independent of any consumer and benefits the codegen dispatcher (#4267), the regex CometUDF (#4239), and the JSON CometUDF (#4305) equally.

The current process-wide ConcurrentHashMap<String, CometUDF> requires every CometUDF to be strictly stateless: one shared instance services all tasks. A thread-local cache would not help because Tokio work-stealing on the scan-free execution path can move a Spark task's future between workers across batches, losing per-batch state. Keying by Spark task attempt ID gives continuity within a task and isolation across tasks regardless of which worker is polling.

What changes are included in this PR?

• CometUdfBridge.INSTANCES becomes ConcurrentHashMap<Long, ConcurrentHashMap<String, CometUDF>> keyed by (taskAttemptId, className).
• A TaskCompletionListener registered on the first cache miss for a task evicts the per-task entry on task end.
• NO_TASK_ID = -1L sentinel covers calls without a TaskContext (unit tests, direct native driver runs); that bucket is not evicted because no task-completion event fires.
• CometUDF Scaladoc updates the contract to "may hold per-task state in fields" and documents the single-threaded-per-instance invariant (Spark runs one native future per partition, Tokio polls one future per worker at a time).
• Defensive assertions on evaluate preconditions, the post-install TaskContext invariant, and the cache-side invariants (single listener registration, non-null cache, reflective-instantiate success).

How are these changes tested?

No new tests in this PR for the same reason as #4306: the Arrow shading boundary in common/ blocks unit tests that subclass CometUDF. End-to-end coverage lands with each consumer (#4267, #4239, #4305) when it drives the bridge.