fix: reject a message whose Transfer-Encoding value cannot be parsed

[pjfanning]

Motivation

When a modelled header value fails to parse, ModeledHeaderValueParser calls onIllegalHeader — which by default only logs — and degrades the header to a RawHeader:

case HeaderParser.Failure(error) =>
  onIllegalHeader(error.withSummaryPrepended(s"Illegal '$headerName' header")...)
  RawHeader(headerName, trimmedHeaderValue)

For Transfer-Encoding that is unsafe. The degraded header never reaches the case h: Transfer-Encoding arm in parseHeaderLines, so isChunked stays false and — with a Content-Length also present — the message is framed by Content-Length. An upstream that does parse the value (stripping quotes, tolerating trailing junk) frames the same message as chunked. The two then disagree about where the message ends, which is a request-smuggling primitive.

Pekko HTTP already rejects the clear cases: chunked together with a Content-Length, an unsupported coding, and multiple entries. Only the unparseable value was silently tolerated.

Modification

Fail the message when a Transfer-Encoding header arrives as a RawHeader. That can only happen when the modelled parse failed: transfer-encoding is in alwaysParsedHeaders, so it is modelled even when modeled-header-parsing is off, and a well-formed value always reaches the modelled arm. The change is therefore scoped to this one header and does not alter the general illegal-header contract.

Result

A message whose Transfer-Encoding cannot be understood is rejected, rather than framed by a different rule than the sender used.

Tests

• sbt "http-core/testOnly org.apache.pekko.http.impl.engine.parsing.*" — pass (246 tests). A new test sends Transfer-Encoding: "chunked" alongside a Content-Length and expects a 400. Verified it fails without the change, where the message is accepted and framed by Content-Length.
• sbt http-core/mimaReportBinaryIssues — pass.
• Native scalafmt clean.

References

None - rejects a message whose Transfer-Encoding value cannot be parsed

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