Fix remainingStreamingBytes timing: set before requestHandler, decrement before dataHandler

[Copilot]

maxRemainingBodyLength() relied on remainingStreamingBytes internally, but the value was set after requestHandler and decremented after dataHandler, making it meaningless when called from either callback.

Changes

• Set remainingStreamingBytes before requestHandler: all three cases (chunked → STATE_IS_CHUNKED, content-length → parsed value, no body → explicit 0) are now resolved prior to the request handler being invoked
• Decrement remainingStreamingBytes before dataHandler: all four content-length streaming call sites in fenceAndConsumePostPadded and consumePostPadded (including post-fallback paths) now update the counter first, compute fin from the pre-decrement value, then invoke dataHandler

// content-length path — before:
dataHandler(user, std::string_view(data, emittable), emittable == remainingStreamingBytes);
remainingStreamingBytes -= emittable;

// after:
bool fin = (emittable == remainingStreamingBytes);
remainingStreamingBytes -= emittable;
dataHandler(user, std::string_view(data, emittable), fin);

Chunked-encoding paths are unaffected — ChunkIterator maintains remainingStreamingBytes internally.

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