[iceberg] Warn when the Iceberg metadata committer factory is missing from the classpath

[bube]

Purpose

Closes #9157.

When metadata.iceberg.storage requests a sync to an external catalog but the committer factory cannot be discovered, Paimon skipped the sync silently. IcebergCommitCallback caught the FactoryException and discarded it, leaving metadataCommitter null so commitToExternalCatalog returned early on every commit. Commits kept succeeding and metadata files kept being written, so the job looked healthy while nothing reached the catalog. In our case a shaded jar had dropped the META-INF/services entry for IcebergRESTMetadataCommitterFactory.

This logs a WARN instead:

No IcebergMetadataCommitterFactory for 'metadata.iceberg.storage=rest-catalog' found on
the classpath, so table mydb.t will not be synced to the external catalog (commits and
metadata files are unaffected). Check that the module providing it is deployed and that
its META-INF/services/org.apache.paimon.factories.Factory entry survived shading.
Cause: Could not find any factories that implement
'org.apache.paimon.iceberg.IcebergMetadataCommitterFactory' in the classpath.

disabled, table-location and hadoop-catalog have no committer factory by design and must stay quiet. IcebergOptions.StorageType now answers that question instead of a switch in the caller:

HIVE_CATALOG("hive-catalog", "...", true),
REST_CATALOG("rest-catalog", "...", true);

It is a required constructor argument, so a new storage type cannot be added without stating its own answer. A committerFactoryIdentifier() accessor replaces the incidental use of toString() as the SPI lookup key. Both factories already derived their identifier from this enum, so this only names the contract.

Still a warning and not a failure, so jobs in this state will not start erroring on upgrade.

Tests

No new test. Reproducing a missing SPI registration inside a module that has one needs classloader manipulation that would test the harness more than the code. Verified by running it across every StorageType:

metadata.iceberg.storage	warns?
table-location	no
hadoop-catalog	no
hive-catalog	yes
rest-catalog	yes

Existing suites pass on JDK 11:

• paimon-core: IcebergCommitCallbackTest 28/28, IcebergCompatibilityTest 40/40
• paimon-iceberg: 30 unit, 4 IT including IcebergRestMetadataCommitterITCase
• paimon-hive-catalog: 62 unit, 14 IT

spotless, checkstyle, enforcer and rat are clean.

API and Format

No format change. StorageType gains two public accessors. Constants, values and descriptions are unchanged, so no config or generated-docs change.

Documentation

None needed.