GH-782: Preserve column nullability in Flight SQL JDBC metadata

[mjuarros]

What's Changed

• Preserve Arrow field nullability when converting Flight SQL result fields into JDBC column metadata, using the JDBC columnNullable and columnNoNulls values.
• Cover nullable and non-nullable columns in both converter tests and the JDBC prepared-statement metadata path.

Verification

• mvn -o -pl :flight-sql-jdbc-core test (JDK 17): 1,247 tests run, 0 failures, 0 errors, 44 skipped.
• mvn -o -pl :flight-sql-jdbc-core -Dtest=ConvertUtilsTest,ArrowFlightPreparedStatementTest -Dsurefire.failIfNoSpecifiedTests=false test (JDK 17, after formatting): 16 run, 0 failures, 0 errors, 1 pre-existing skip.
• mvn -o -pl :flight-sql-jdbc-core spotless:check (JDK 21): passed.
• Standalone JDBC smoke check against a local Derby-backed Flight SQL example: ID=0 (not nullable), KEYNAME=1 (nullable) from PreparedStatement.getMetaData().

Closes #782.

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