Use specific Utf8Error for Message::{into_text, to_text}

[sehyunsix]

Summary

• Change Message::into_text() return type from Result<Utf8Bytes> to Result<Utf8Bytes, Utf8Error>
• Change Message::to_text() return type from Result<&str> to Result<&str, Utf8Error>
• These methods can only produce UTF-8 errors, so this aligns them with Frame::into_text() and Frame::to_text() which already use specific error types

Closes #438

Notes

This is a breaking change for callers that explicitly annotate the error type as tungstenite::Error. However, callers using ? in functions returning tungstenite::Result will continue to work since From<Utf8Error> for tungstenite::Error is already implemented.

Test plan

• cargo check passes
• All 43 unit tests pass
• All 12 integration tests pass
• All 4 doc-tests pass

🤖 Generated with Claude Code

[daniel-abramov]

I agree it would be better to have some consistency in the error types.

I wonder if we could simply change the type of Error::Utf8(String) to Error::Utf8(Utf8Error). To my memory, the last change in this direction was done in 255aaa2. Perhaps we could just use Utf8Error instead? That would unify the error's return type across the crate. I think we use our own Result<X, Error> pretty much everywhere in the public API, except probably in parts of Frame (which is arguably a bit lower-level).