Fix checkpoint with SET-based credentials

[pckilgore]

fixes #1222

Copies users context through some new connections created when doing Checkpoint to ensure settings are available when needed:

User's session (ClientContext A)
  has: SET s3_access_key_id = 'admin', SET s3_secret_access_key = 'password', etc.
  runs: CHECKPOINT;

then

DuckLakeTransactionManager::Checkpoint(context=A)
  creates: Connection #1 (new ClientContext B — empty settings)
  ───── FIX #1: B.user_settings = A.user_settings ─────
  runs: conn->Query("CALL ducklake_delete_orphaned_files('ducklake')")

then

ducklake_delete_orphaned_files executes
  needs to query DuckLake metadata + S3
  calls: DuckLakeTransaction::Get(context=B, catalog)
         gets/creates a DuckLakeTransaction where weak_ptr context == B

then

DuckLakeTransaction::GetConnection()
  creates: Connection #2 (new ClientContext C — empty settings)
  ───── FIX #2: C.user_settings = B.user_settings ─────
  runs metadata query including: read_blob 

then
       
httpfs resolves S3 credentials
  calls ClientContext C → TryGetCurrentSetting("s3_access_key_id")
  looks in: C.config.user_settings
  WITHOUT fixes: empty == 403 Forbidden
  WITH fixes: finds 'admin' (copied A to B to C) == success

Tests all pass locally.