[CELEBORN-XXXX][WIP] Mark CelebornShuffleHandle as reliably stored for per-shuffle DRA safety

[venkata91]

What changes were proposed in this pull request?

• Adds a per-shuffle reliable-storage signal so Spark does not recompute a lost executor's map stages for shuffles whose data lives on Celeborn workers.
• On executor loss, Spark's DAGScheduler unregisters the executor's map outputs and recomputes the map stage. A CelebornShuffleHandle is only created for shuffles stored on Celeborn workers (fallback shuffles use Spark's SortShuffleManager), so their output survives executor loss and the recompute is wasteful.
• Overrides ShuffleHandle.isReliablyStored to return true on CelebornShuffleHandle; fallback shuffles keep Spark's BaseShuffleHandle (default false), so the mixed case is handled per shuffle.
• The override is committed commented out: ShuffleHandle.isReliablyStored does not exist in any released Spark, so enabling it would break compilation against the Spark versions Celeborn builds against.
• Blocked on the Spark-side PR ([SPARK-59138][CORE] Preserve reliably-stored shuffle map outputs on executor loss spark#58437, SPARK-59138). Do not merge until that lands, ships in a release, and Celeborn's Spark dependency is bumped; then uncomment the override and update the JIRA IDs.
• Follow-up to CELEBORN-2451, which ties supportsReliableStorage to the NEVER fallback policy to stay DRA-safe. This PR lets AUTO fallback avoid the recompute penalty per shuffle.

Why are the changes needed?

• Under DRA with AUTO fallback, losing an executor needlessly recomputes map stages whose output is safely stored on Celeborn workers.
• The app-global supportsReliableStorage() flag cannot express "this shuffle is on Celeborn, that one fell back to local disk," so it forces NEVER to be safe. Per-shuffle reliability removes that constraint.

Does this PR resolve a correctness bug?

• Yes

Does this PR introduce any user-facing change?

• Yes

How was this patch tested?

• Not yet exercised end-to-end: the override is commented out pending the Spark dependency.
• Once unblocked, validated against a Spark build carrying SPARK-59138 with a DRA + AUTO-fallback job that loses an executor, asserting the Celeborn-backed shuffle's map stage is not recomputed while a fallback shuffle's is.