Pre-release model versions should warn (not error) on contract breaking changes

[jecolvin]

Summary

Resolves #12164

• Pre-release model versions (where version > latest_version) now produce a warning instead of a hard ContractBreakingChangeError when a breaking contract change is detected
• Applies to both contract modifications (same_contract) and model deletion/disabling (same_contract_removed)
• Non-prerelease versioned models continue to raise errors as before
• Users can restore the old strict behavior via --warn-error or --warn-error-options

Implementation notes

Adds an is_prerelease property to ModelNode that reuses the existing version comparison logic from VersionSelectorMethod. The two decision points in same_contract() and same_contract_removed() are updated from a 2-way branch (unversioned→warn, versioned→error) to a 3-way branch (unversioned→warn, prerelease→warn, non-prerelease versioned→error).

dbt-protos workaround: The event/logging system requires protobuf messages defined in the external dbt-protos package, which cannot be modified from within dbt-core. Rather than introducing a new PrereleaseBreakingChange event type (which would require a corresponding proto message), this PR reuses the existing UnversionedBreakingChange event with "(pre-release)" appended to the model name to clearly identify the source.

Test plan

• New TestChangedContractPrereleaseVersioned — makes a breaking change only to a pre-release version (v2), verifies no ContractBreakingChangeError is raised
• New TestDeletePrereleaseVersionedContractedModel — deletes a pre-release version, verifies warning instead of error
• All 46 existing test_modified_state integration tests pass
• All 1610 unit tests pass
• Linting (flake8 + mypy) passes

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