Add support for SSE comments

[magdzikk]

Fixes #379.

ServerSentEvent gains a comments: List[String] field, so comment lines (those starting with :) round-trip through both parse and toString. ServerSentEvent.comment("ping") builds the keep-alive frame the WhatWG specification recommends sending every 15 seconds or so, to stop legacy proxies from dropping an idle connection.

Design notes

List[String] rather than Option[String], because a single event block may carry several comment lines.

There is deliberately no assertion that a comment and data aren't both set (as considered in the issue discussion). The specification ignores comments per line, not per event, so a block such as

: keep-alive
data: abc

is valid and still dispatches abc. A guard would reject valid streams.

Binary compatibility

Preserved with the same approach already used for ContentTypeRange: the old-arity constructor, copy and apply are kept alongside the new ones, and mimaReportBinaryIssues is green.

MiMa does not see everything, though. The source-level and runtime-shape changes it cannot report are listed under Migration below.

Migration

Everything here follows from ServerSentEvent gaining a field. Recompiling against the new version is the general fix; the two behaviour changes need a code change as well.

Comment-only events are no longer empty. A : ping block used to parse to ServerSentEvent(), and now parses to an event with comments set, so code that dropped keep-alives by comparing against an empty event stops dropping them:

events.filterNot(_ == ServerSentEvent())  // no longer drops keep-alives
events.filterNot(_.hasNoFields)           // use this instead

hasNoFields is true when an event carries no data, event type, id or retry, so it drops exactly the set that the old comparison dropped — keep-alives, empty blocks, and blocks of unknown fields alike.

Events parsed from a stream containing comments no longer equal hand-built events. Streams often open with a : handshake comment, so expected values need updating, or the comments dropped before comparing:

ServerSentEvent.parse(lines) == ServerSentEvent(Some("x"))                      // false once a comment is present
ServerSentEvent.parse(lines).copy(comments = Nil) == ServerSentEvent(Some("x")) // compare without them

Patterns over the four original fields no longer compile. Add a fifth _, or use the accessors:

case ServerSentEvent(d, e, i, r)    => ... // does not compile
case ServerSentEvent(d, e, i, r, _) => ...

Code compiled against an earlier version has to be recompiled. The changes below leave the erased signatures intact, so such code links and then fails at runtime, and MiMa cannot report it:

• ServerSentEvent.unapply now returns a Tuple5. A caller compiled earlier fails with ClassCastException: scala.Tuple5 cannot be cast to scala.Tuple4. Compiled patterns are unaffected — they lower to accessor calls rather than to unapply.
• Scala 3 codecs derived through Mirror read a fifth element. One compiled earlier fails with IndexOutOfBoundsException: 4.

Serialised forms change.

• serialVersionUID changes from 5877847238202557287 to -2395116303002558390, so Java-serialised events do not cross the version boundary.
• JSON codecs derived from the case class gain a comments field. circe and play-json ignore Scala default arguments, so their derived decoders reject JSON that lacks it; jsoniter-scala and zio-json read the default and are unaffected.

[adamw]

Automated review by Claude (posted by @adamw). Main finding: comments are not split on newlines in toString. Details inline.