Verify unreachable code block in BalsaFrame::ProcessHeaderLines.

- Add `QUICHE_CODE_COUNT` to test reachability of the block handling chunked transfer encoding when Content-Length is present.
- Add assertion in `BothTransferEncodingAndContentLengthAllowed` unit test to prove Content-Length is NOT cleared, validating that the block is unreachable in standard flows.

PiperOrigin-RevId: 967324169

quiche/balsa/balsa_frame.cc

diff --git a/quiche/balsa/balsa_frame.cc b/quiche/balsa/balsa_frame.cc
index 747711b..af17add 100644
--- a/quiche/balsa/balsa_frame.cc
+++ b/quiche/balsa/balsa_frame.cc
@@ -899,6 +899,10 @@
     }
 
     if (headers->transfer_encoding_is_chunked_) {
+      // Verify this code block is never reached as
+      // headers->transfer_encoding_is_chunked_ is supposed to be false before
+      // the call to ProcessTransferEncodingLine.
+      QUICHE_CODE_COUNT(chunked_transfer_encoding_detected);
       headers->content_length_ = 0;
       headers->content_length_status_ = BalsaHeadersEnums::NO_CONTENT_LENGTH;
       content_length_remaining_ = 0;