fix(rest): release fetched FileScanTask state to prevent unbounded memory growth

[Solaris-star]

Summary

Fixes #17427

The reference REST server-side scan planning implementation retains all planned FileScanTask objects in the singleton InMemoryPlanningState even after clients have successfully fetched every plan task. Neither fileScanTasksForPlanTask nor nextPlanTask removes the fetched entries, and the state is only released via cancelPlan. Per the REST Catalog OpenAPI spec, cancellation is not required after all plan tasks have been fetched, so a successful fetch lifecycle must release state without an explicit cancel request.

Fix

Add two methods to InMemoryPlanningState:

• releasePlanTask(planTaskKey) — removes a single fetched plan task's FileScanTask list and next-task link
• releaseAsyncPlanForTask(planTaskKey) — removes the async planning state when the last plan task in a chain is fetched (extracts the planId from the key format {planId}-{tableId}-{sequence})

Call both from CatalogHandlers.fetchScanTasks after building the response:

IN_MEMORY_PLANNING_STATE.releasePlanTask(planTask);
if (nextPlanTasks.isEmpty()) {
  IN_MEMORY_PLANNING_STATE.releaseAsyncPlanForTask(planTask);
}

Tests

Added TestInMemoryPlanningState with 4 unit tests:

• releasePlanTask removes both file scan tasks and next-task link
• releaseAsyncPlanForTask removes async planning state
• malformed keys are handled gracefully (no exception, no side effects)
• releasing unknown keys is a no-op

Note: I could not run the full Gradle test suite locally (partial clone conflicts with the multi-module build), but the changes are minimal Map.remove() calls following existing patterns in the codebase. CI should validate.