[improvement] Guard schema changes against live Global Index references (#9419)

[zhang-arvin]

Purpose

This PR adds a guard in SchemaManager that prevents schema changes (specifically DropColumn) from being applied when the column is referenced by a live Global Index.

Problem

Currently, schema changes like dropping a column can proceed even when Global Indexes defined on that column exist. This leads to inconsistent index state and potential data corruption.

Solution

Added checkGlobalIndexConflicts method in SchemaManager that:

1. Scans table options for global-index entries to extract indexed column names
2. Before applying schema changes, checks if any DropColumn targets a Global Index column
3. Throws UnsupportedOperationException with a clear message directing users to drop the Global Index first

Testing

• Compilation verified: mvn compile -pl paimon-core -am -DskipTests passes

Related Issue

Closes #9419