Make DuckDB attachments logic more robust

[sgrebnov]

Note: draft is I found out that DuckDB does not recognize created tables after file was attached so this should be done when all accelerated tables are available.

Fixes DuckDB errors caused by a race condition when multiple connections attempt to attach databases concurrently.

When multiple connections called query_arrow() simultaneously, each would:

1. Check if attachments exist via PRAGMA database_list
2. If not found, create new DuckDBAttachments with a unique random ID
3. Run ATTACH IF NOT EXISTS '{db}' AS attachment_{random_id}_{i} one by one

Race condition: Between step 1 (check) and steps 3 (attach one by one), another connection could attach the same file or retrieve only partially attached databases. This result into errors

Spice query failed. Status: 400, body: Execution error: Failed to execute query.\nDuckDB connection failed.\nBinder Error: Unique file handle conflict: Cannot attach "attachment_SDFrVyz5_0" - the database file "/app/.spice/data/accelerated_duckdb.db" is already attached by database "attachment_23yXahuv_0"\nFor details, refer to the DuckDB manual: https://duckdb.org/docs/"}

2025-11-17T19:30:47.876652Z WARN datafusion_table_providers::sql::db_connection_pool::dbconnection::duckdbconn: my_table.duckdb not found among existing attachments

All DuckDB connections acquired from a single pool or created via try_clone() share the same catalog including attached databases, but not the search_path which is connection-level setting. PR updates attachment logic to attach databases once during pool initialization and use pre-computed search path for individual connections setup (`SET search_path = '...'] is a fast, idempotent session-level command)

┌─────────────────────────────────────────────────────────┐
│                  duckdb_database (self.db)              │
│  ┌─────────────────────────────────────────────────┐    │
│  │              Catalog (shared state)              │    │
│  │  - Tables                                        │    │
│  │  - Attached databases (attachment_xxx_0, etc.)   │    │
│  │  - Search paths (per-connection setting)         │    │
│  └─────────────────────────────────────────────────┘    │
│                                                         │
│  ┌──────────────┐  ┌──────────────┐  ┌──────────────┐   │
│  │ Connection 1 │  │ Connection 2 │  │ Connection 3 │   │
│  │  (original)  │  │ (try_clone)  │  │ (pool.get())  │   │
│  │              │  │              │  │              │   │
│  │ search_path: │  │ search_path: │  │ search_path: │   │
│  │   "main"     │  │   (default)  │  │   (default)  │   │
│  └──────────────┘  └──────────────┘  └──────────────┘   │
└─────────────────────────────────────────────────────────┘

[sgrebnov]

Closed in favor of #509