Decouple maxRemainingBodyLength() from live remainingStreamingBytes state

[Copilot]

maxRemainingBodyLength() was reading directly from remainingStreamingBytes, which is mutated during parsing and doesn't reflect a stable "remaining after this chunk" value at the point a dataHandler callback fires.

Changes

• New member remainingBodyLength: A separate uint64_t remainingBodyLength = 0 field in HttpParser, decoupled from the internal parsing state variable.
• Set before every dataHandler call (and nowhere else):
  • Content-length: remainingBodyLength = remainingStreamingBytes - emittedChunkSize — bytes left after the current chunk
  • All-remaining / final chunk: remainingBodyLength = 0
  • Chunked (non-final): remainingBodyLength = STATE_IS_CHUNKED — preserves UINT64_MAX semantics in maxRemainingBodyLength()
  • Chunked (final empty chunk) / empty body: remainingBodyLength = 0
• maxRemainingBodyLength() reads from remainingBodyLength instead of remainingStreamingBytes

uint64_t maxRemainingBodyLength() {
    if (isParsingChunkedEncoding(remainingBodyLength)) {
        return UINT64_MAX;
    }
    return remainingBodyLength;
}

This means callers invoking maxRemainingBodyLength() inside a dataHandler callback now see the remaining body length after the currently emitted chunk is accounted for, not the raw internal streaming counter.

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