feat: opt-in create_if_not_exists for concurrent table creation

[JoepBeurskens]

Summary

Fixes #721. See CHANGELOG. Adds an opt-in create_if_not_exists model config; default off; not for Replicated engines.

Checklist

Delete items not relevant to your PR:

• Unit and integration tests covering the common scenarios were added
• A changelog entry was added following Updating the Changelog (user-facing, concise, PR link)
• For significant changes, documentation in https://github.com/ClickHouse/clickhouse-docs was updated with further explanations or tutorials

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[Copilot]

Pull request overview

Adds opt-in atomic table creation intended to mitigate concurrent first-creation races.

Changes:

• Adds create_if_not_exists SQL generation and compatibility checks.
• Adds integration coverage for enabled and default behavior.
• Documents the feature in the changelog.

Reviewed changes

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

File	Description
table.sql	Implements atomic conditional table creation.
test_create_if_not_exists.py	Tests generated SQL and row counts.
CHANGELOG.md	Adds the user-facing feature entry.

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