[Fix][Connector-V2][Clickhouse] Fix ClickHouse catalog nullable column type and add tests

[zhangshenghang]

Purpose of this pull request

Does this PR introduce any user-facing change?

How was this patch tested?

Check list

• If any new Jar binary package adding in your PR, please add License Notice according
  New License Guide
• If necessary, please update the documentation to describe the new feature. https://github.com/apache/seatunnel/tree/dev/docs
• If necessary, please update incompatible-changes.md to describe the incompatibility caused by this PR.
• If you are contributing the connector code, please check that the following files are updated:
  1. Update plugin-mapping.properties and add new connector information in it
  2. Update the pom file of seatunnel-dist
  3. Add ci label in label-scope-conf
  4. Add e2e testcase in seatunnel-e2e
  5. Update connector plugin_config

[Copilot]

Pull request overview

This PR fixes the ClickHouse connector to properly handle nullable column types when creating tables. The fix ensures that:

1. Nullable columns are correctly wrapped with Nullable() type
2. Primary key columns are never wrapped with Nullable() (as ClickHouse doesn't allow nullable columns in ORDER BY/PRIMARY KEY)
3. Map and Array types are excluded from nullable wrapping (ClickHouse limitation)

Key changes:

• Added ThreadLocal-based tracking of primary key columns during table creation
• Enhanced columnToConnectorType to wrap nullable columns with Nullable() while respecting ClickHouse constraints
• Added comprehensive unit tests for nullable handling and comment escaping
• Updated integration test assertion to include stdout for better debugging

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
ClickhouseCatalogUtil.java	Core fix: Added nullable type wrapping logic with primary key and complex type exclusions via ThreadLocal tracking
ClickhouseCatalogUtilTest.java	Added unit tests for nullable wrapping, comment escaping, and null input handling
ClickhouseCreateTableTest.java	Updated expected SQL to reflect Nullable() wrapping for non-primary-key columns
ClickhouseIT.java	Enhanced test assertion to include stdout in error messages for debugging

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