fix: Keep the write error in the chain of Encoder errors

[keelerm84]

Encoder formatted the underlying write error with %v, which turns it into
text. A caller could not use errors.Is or errors.As on the result, so it
could not tell a write deadline cut (os.ErrDeadlineExceeded) from a reset
or a broken pipe without matching the message. The error now wraps with
%w. The message text is unchanged.

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Note

Overview
Encoder write failures now preserve the underlying error so callers can use errors.Is / errors.As (e.g. to distinguish os.ErrDeadlineExceeded from connection resets) instead of parsing the message string.

All Encode paths that return "eventsource encode: …" now wrap the io.WriteString error with %w instead of %v; the user-facing message text is unchanged.

Adds TestEncoderKeepsTheWriteErrorInTheChain, covering plain events, comments, and gzip-compressed encoding via a stub writer that always fails.

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