Add RFC 2047 non-ASCII header encoding/decoding support (#405)

* Add RFC 2047 encoding/decoding support for non-ASCII header values

Co-authored-by: Nugine <30099658+Nugine@users.noreply.github.com>

* Address code review feedback for RFC 2047 implementation

Co-authored-by: Nugine <30099658+Nugine@users.noreply.github.com>

* Focus PR on rfc2047 module only and add comprehensive tests for 100% coverage

Co-authored-by: Nugine <30099658+Nugine@users.noreply.github.com>

* Improve test comment clarity

Co-authored-by: Nugine <30099658+Nugine@users.noreply.github.com>

* Use Cow to reduce allocations and thiserror for error definitions

Co-authored-by: Nugine <30099658+Nugine@users.noreply.github.com>

* Apply suggestions from code review

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Enforce RFC 2047 75-character limit for encoded-words

Co-authored-by: Nugine <30099658+Nugine@users.noreply.github.com>

* Fix UTF-8 character boundaries and whitespace handling in RFC 2047

Co-authored-by: Nugine <30099658+Nugine@users.noreply.github.com>

* Apply review feedback: safety assertions and empty result check

Co-authored-by: Nugine <30099658+Nugine@users.noreply.github.com>

* Apply comprehensive review feedback: security, optimizations, and testing improvements

Co-authored-by: Nugine <30099658+Nugine@users.noreply.github.com>

* Add input size validation to decode and comprehensive EncodeError tests

Co-authored-by: Nugine <30099658+Nugine@users.noreply.github.com>

* fix

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Nugine <30099658+Nugine@users.noreply.github.com>
Co-authored-by: Nugine <nugine@foxmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>