Add warning log to EvictionQueue::Purge for slow purge detection

[krleonid]

Summary

• Add DUCKDB_LOG_WARNING to EvictionQueue::Purge() when iterations > 10 or elapsed time > 1 second
• Logs queue_size and dead_nodes count for diagnosis
• Also prints to stderr in DEBUG builds via Printer::PrintF

Usage

SET GLOBAL enable_logging = true;
SET GLOBAL logging_level = 'warning';
-- run workload, then:
SELECT * FROM duckdb_logs() WHERE message LIKE '%Purge%';

Test plan

• Run concurrent workload with multiple attached databases
• Verify warning appears in duckdb_logs() when purge takes many iterations
• Verify no output when purge is fast (< 10 iterations, < 1s)

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