[server] Fix NPE when processing table registration change

[raoluSmile]

What is the purpose of the change

This PR fixes a NullPointerException in
CoordinatorEventProcessor.processTableRegistrationChange.

When processing a TableRegistrationChangeEvent, the coordinator needs the old
TableInfo to get the current schema info and build the new TableInfo.
However, the old TableInfo may be unavailable if the event is stale or the
coordinator context is incomplete.

The previous code only checked whether tableId was null, but
CoordinatorContext#getTableIdByPath returns TableInfo.UNKNOWN_TABLE_ID when
the table path is not found. As a result, an unknown table could still continue
to getTableInfoById, get null, and fail at oldTableInfo.getSchemaInfo().

Brief change log

• Treat TableInfo.UNKNOWN_TABLE_ID as an unregistered table when processing
  table registration change events.
• Skip table registration change events if the old TableInfo is not available
  in CoordinatorContext.
• Add regression tests for stale/incomplete coordinator context states.

Tests

Added two regression tests in CoordinatorEventProcessorTest:

• testTableRegistrationChangeWithUnknownTable

  Covers the case where a table registration change event is processed after the
  table path is no longer present in CoordinatorContext.

  CoordinatorContext#getTableIdByPath returns TableInfo.UNKNOWN_TABLE_ID
  for this case. The previous tableId == null check did not catch it, so the
  processor continued and eventually dereferenced a null oldTableInfo.

• testTableRegistrationChangeWithMissingTableInfo

  Covers the case where the table path to table id mapping exists, but the
  corresponding TableInfo is not available in CoordinatorContext.

  This represents an incomplete coordinator context state. Since
  TableRegistration#toTableInfo needs the old schema info, the event cannot be
  applied safely without the old TableInfo.

Before this fix, both tests failed with an NPE at
oldTableInfo.getSchemaInfo(). After this fix, the stale/incomplete events are
skipped safely.

./mvnw -pl fluss-server -am \
  -Dtest=CoordinatorEventProcessorTest#testTableRegistrationChangeWithUnknownTable+testTableRegistrationChangeWithMissingTableInfo \
  -DfailIfNoTests=false test

Tests run: 2, Failures: 0, Errors: 0, Skipped: 0
BUILD SUCCESS

[loserwang1024]

Thanks for your pr, I have left some comment.