Revert chunked encoding return semantics to restore parser correctness

[Copilot]

The chunked transfer parser was emitting end-of-body markers at the wrong point after a refactor, which altered the terminal-state semantics for chunked and trailer parsing. This restores the prior behavior so the parser continues to signal completion only when the framing state actually reaches the correct end-of-body condition.

• What changed

  • Reverted the return semantics introduced in e7deb72 in src/ChunkedEncoding.h.
  • Restored the original std::nullopt terminal behavior for the trailer final-empty-line and legacy fallback paths.
  • Preserved the empty-terminated chunk signal only in the legacy state transition where it was intentionally used before the refactor.

• Why this matters

  • The parser must not report “end of body” prematurely while still inside trailer parsing or while final chunk framing is incomplete.
  • The reverted behavior keeps chunk termination aligned with the HTTP chunked framing rules and avoids false completion during trailer handling.

• Example

if (c == '\n') {
    state = 0;
    return std::nullopt;  // report completion only when the state is actually terminal
}

• Impact
  • Restores the parser behavior expected by the surrounding HTTP handling code.
  • Keeps chunked-body termination deterministic across partial-buffer and trailer edge cases.