fix: make Resend API errors picklable

[nickita-khylkouski]

Summary

Define ResendError#__reduce__ so API errors can be pickled without calling subclass constructors with incomplete Exception.args. The round trip preserves the concrete exception type, args, message, headers, and error metadata.

Testing

• Full pytest suite: 674 passed
• mypy: 181 source files, no issues
• flake8 passed under the repository's supported Python 3.11 environment
• Package build passed

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Summary by cubic

Makes ResendError picklable by adding __reduce__, so API errors can be serialized and restored with the same concrete type, args, message, headers, and error metadata. Previously pickling could reconstruct errors with incomplete Exception.args, losing custom state.

Adds pickle round-trip tests across all supported pickle protocols and common error type/code combinations.

^{Written for commit 31a7dc4. Summary will update on new commits.}