fix(duckdb): use SET GLOBAL for all settings to ensure pool consistency

[khalid244]

Summary

Change all SET statements to SET GLOBAL to ensure settings apply to all connections in the pool.

Arc uses a connection pool (SetMaxOpenConns), so SET only affects one connection while other pooled connections use defaults.

Changes

• SET memory_limit → SET GLOBAL memory_limit
• SET threads → SET GLOBAL threads
• SET enable_object_cache → SET GLOBAL enable_object_cache
• SET preserve_insertion_order → SET GLOBAL preserve_insertion_order
• SET cache_httpfs_type → SET GLOBAL cache_httpfs_type
• SET cache_httpfs_max_in_mem_cache_block_count → SET GLOBAL ...
• SET cache_httpfs_in_mem_cache_block_timeout_millisec → SET GLOBAL ...
• SET custom_profiling_settings → SET GLOBAL custom_profiling_settings

Test plan

• Build passes
• All database tests pass
• No functional change, just scope correction

[xe-nvdk]

Good catch on the connection pool issue — SET only applies to the connection that runs it, so other pooled connections would use defaults. SET GLOBAL is the right fix for most of these.

Two issues:

1. custom_profiling_settings should stay SET, not SET GLOBAL

QueryWithProfileContext is meant to enable profiling for a specific diagnostic query. Making it SET GLOBAL enables profiling on all connections, adding overhead to every query — not just the one being profiled. This should remain SET (or ideally be scoped to just that connection/transaction).

2. Conflicts with PR #171

This PR still references enable_object_cache (line 140), but PR #171 replaces it with parquet_metadata_cache. These two PRs will conflict — please coordinate with #171 and rebase after it merges.

[xe-nvdk]

Thanks for catching the connection pool scoping issue, Khalid — this is a solid fix. We'll handle the rebase against #171 on our end and apply the remaining SET GLOBAL changes to the new settings. Merging as-is and fixing up locally.