Attach DuckDB connection pool using Kestrel connection API instead of middleware

[qodo-code-review]

Description

• Current implementation uses middleware to attach lazy DuckDB connection pools to HTTP connections, adding unnecessary complexity to the request pipeline
• Pool instantiation is cheap (only parses connection string, no IO), making lazy initialization unnecessary
• Kestrel provides a more suitable API for managing per-connection state through connection context items

Deliverables

• Remove middleware: Delete DuckDbConnectionPoolMiddleware and its registration from the startup pipeline
• Implement Kestrel API: Add connection-level pool attachment using ListenOptions.Use() in KestrelHelpers
• Update references: Change all Lazy<DuckDBConnectionPool> parameters to DuckDBConnectionPool throughout index read operations
• Add extension: Create HttpContextExtensions.GetDuckDbConnectionPool() for retrieving pools from HTTP context

[linear]

DB-1800