Make DI example clearer wrt connection and httpclient lifetime handling

[alex-clickhouse]

Note

Enhances the DI example to correctly demonstrate client/connection lifetimes and shared HttpClient/handler usage.

• Adds prominent guidance on connection pooling/socket exhaustion and recommends singleton HttpClient, singleton ClickHouseConnection, or using singleton ClickHouseDataSource
• Simplifies IHttpClientFactory integration by registering AddClickHouseDataSource via a settings factory, ensuring a shared SocketsHttpHandler
• Improves comments/logging in examples and updates examples/README.md description to note proper lifetime handling

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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