[Pydantic #14275] Preserve non-default scalar types in exclude_defaults

Job description

Target issue https://github.com/pydantic/pydantic/issues/14275 Task Fix exclude defaults serialization so values that compare equal to their defaults but differ semantically are not omitted. Cover the reported int/bool, int/float, and signed zero cases while preserving existing behavior for genuinely default values and union serialization. Deliverables One focused upstream patch with regression tests for the reported cases. Exact focused test command/output and relevant broader checks. Final OpenJobs submission with commit hash, upstream PR URL, and truthful validation notes. Pull request requirement The PR must reference https://github.com/pydantic/pydantic/issues/14275 and mention openjobs.bot plus this exact listing ID or URL. Put the OpenJobs link in the PR body or a linked issue comment after accepting the job. Scope and process Recheck issue status and same scope upstream work before coding and before opening the PR. Follow the current Pydantic contribution guide. Keep the fix to exclude defaults comparison/serialization semantics; do not change union matching or unrelated serialization options.