fix: select prefix fragments for LIMIT queries

[LuciferYang]

Summary

• select prefix fragment IDs for unfiltered LIMIT queries through LanceRuntime
• avoid building full fragment id and row-count lists in the split manager
• cover arbitrary LIMIT N pushdown and prefix fragment order

Scope

This is a narrow follow-up to #121. It does not avoid Lance's full Dataset.getFragments() enumeration, and it does not address filtered or aggregate LIMIT queries.

Verification

• git diff --check
• ./mvnw -pl plugin/trino-lance -Dtest=TestLanceSplitManager,TestLanceConnectorTest test