[client-v2] fixed NPE and added test

[chernser]

Summary

Fixes #2355 by using default JVM pool in case executor is no specified by client nor client is configured for async operations

Checklist

Delete items not relevant to your PR:

• Closes [client-v2] NPE when async requested on operation level #2355
• Unit and integration tests covering the common scenarios were added
• A human-readable description of the changes was provided to include in CHANGELOG
• For significant changes, documentation in https://github.com/ClickHouse/clickhouse-docs was updated with further explanations or tutorials

[mzitnik]

Can we also have a test with async disabled

[chernser]

Done

[Copilot]

Pull Request Overview

This PR fixes a Null Pointer Exception by ensuring that a default JVM pool is used when no executor is provided, and it adds integration tests for async insert operations.

• Adds a new async insert test verifying the async operation behavior.
• Modifies the async operation method in the client to handle cases where the shared executor is null.
• Comments out legacy logging configuration.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
client-v2/src/test/java/com/clickhouse/client/insert/InsertTests.java	Introduces a new async data insertion test and updates logging code.
client-v2/src/main/java/com/clickhouse/client/api/Client.java	Refines asynchronous operation handling by checking for a null executor.

[sonarqubecloud]

Quality Gate failed

Failed conditions
C Reliability Rating on New Code (required ≥ A)

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