[AMORO-4264][AMS] Use a dedicated thread pool for snapshot expiration planning

[johntomcat7408-cmyk]

Why are the changes needed?

Snapshot expiration planning uses Iceberg's global worker pool by default. Under load, expiration planning can occupy that shared pool while self-optimizing commits wait for worker futures, leaving tables in the committing state for a long time.

Close #4264.

Brief change log

• Add a process-wide planning pool dedicated to snapshot expiration.
• Configure its size with expire-snapshots.plan-thread-count (default: 10) from the Iceberg process factory.
• Pass the dedicated executor to ExpireSnapshots.planWith(...).
• Add regression coverage for pool isolation, configuration validation, and the expiration call path.

How was this patch tested?

• Add test cases that check positive and negative cases.
• Screenshots are not applicable.
• Run tests locally and on a Linux VM.

Validated on Linux with both JDK 11 and JDK 17:

./mvnw test \
  -pl amoro-ams \
  -am \
  -Pskip-dashboard-build \
  -Dtest=TestIcebergThreadPools,TestIcebergTableMaintainer,TestIcebergProcessFactory,TestSnapshotExpireIceberg \
  -Dsurefire.failIfNoSpecifiedTests=false

Both runs completed with BUILD SUCCESS. The focused unit tests passed without failures or errors, and TestSnapshotExpireIceberg reported 0 failures and 0 errors.

Documentation

• Does this pull request introduce a new feature? No; it isolates an existing maintenance operation and adds a tuning option.
• The default option is included in process-factories.yaml, and the pool scope and fallback behavior are documented in JavaDocs.