Flip default for require_all_warnings_handled_by_warn_error to true

[Fury0508]

• Changed default from false to true in core/dbt/contracts/project.py
• Added changelog entry for issue flip default require_all_warnings_handled_by_warn_error to true #12162

Resolves #12162

Problem

The flag require_all_warnings_handled_by_warn_error was introduced with a default value of false for backward compatibility. As per issue #12162, we need to flip the default to true for the v1.12 milestone to ensure all warnings are properly handled by warn-error configurations.

Solution

Changed the default value of require_all_warnings_handled_by_warn_error from False to True in core/dbt/contracts/project.py.

This is a straightforward boolean flip. Existing tests explicitly set both true and false values, so no test modifications were needed. All unit tests pass (1540 tests) and code quality checks pass.

Users who need the old behavior can explicitly set require_all_warnings_handled_by_warn_error: false in their dbt_project.yml file.

Checklist

• I have read the contributing guide and understand what's expected of me.
• I have run this code in development, and it appears to resolve the stated issue.
• This PR includes tests, or tests are not required or relevant for this PR.
• This PR has no interface changes (e.g., macros, CLI, logs, JSON artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX.
• This PR includes type annotations for new and modified functions.

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