fix: reject whitespace between chunk-size digits (#1257)

[pjfanning]

cherry pick 7e39060 #1257

Motivation:
The chunk-size parser tolerates whitespace after the size digits (illegal per the spec but seen in the wild, see #1812), but the WSP arm recursed with the same accumulated size and then let parsing continue with the next character — including a further hex digit. So a chunk size such as 5 0 was read as 0x50 = 80 bytes, while a proxy that stops at the space reads 5. That discrepancy is a request- smuggling primitive when Pekko HTTP sits behind such a proxy. The comment referencing #1812 intended only trailing whitespace; the implementation skipped interior whitespace too. The WSP arm also had no cursor > offset guard, so a size field of only spaces parsed as 0.

Modification:
Thread a sawWhitespace flag through parseSize. Once whitespace has been seen, a hex digit no longer matches the accumulation arm and falls through to the illegal-character case, so only more whitespace or a terminator may follow. Guard the WSP arm with cursor > offset so leading whitespace (before any digit) is rejected as well.

Result:
Trailing whitespace after the chunk size is still tolerated, but whitespace between size digits, and leading whitespace, are rejected with "Illegal character '...' in chunk start" — Pekko HTTP and a fronting proxy can no longer disagree on the chunk size.

Tests:

• sbt "http-core/testOnly org.apache.pekko.http.impl.engine.parsing.RequestParserCRLFSpec org.apache.pekko.http.impl.engine.parsing.RequestParserLFSpec" - pass (116 tests); two new tests reject 5 0 (interior whitespace) and 5 (leading whitespace), and the existing "incorrect but harmless whitespace after chunk size" test still passes. Verified the interior-whitespace test fails with the fix stashed (5 0 parses as 80 bytes).
• sbt http-core/mimaReportBinaryIssues - pass (internal impl.engine.parsing change, no public API).

References:
Refs #1812 - narrows the whitespace tolerance to trailing whitespace only