redis codec: fix \xHH hex escape crashing on literal 'x' in inline args

[waterWang]

Fixes #46642

Root cause

InlineStringQuotedEscapeHex pushes the escape marker x into the value
string, then scans 3 characters back for the same marker. When the user's
argument already contains a literal x before a \x hex escape, the 3-back
test matches the literal x instead of the escape marker, and std::stoul
throws std::invalid_argument on an invalid hex string. The exception
escapes the filter chain (only Redis::ProtocolError is caught) and reaches
std::terminate, aborting the entire Envoy process.

Fix

1. codec_impl.h — add inline_hex_digit_count_ member to track hex
   digits seen in the current \xHH escape.
2. codec_impl.cc — stop pushing the escape marker into the value
   string. Track hex-digit count instead of scanning 3-back. After exactly two
   hex digits, convert and replace them with the decoded byte, matching Redis
   \xHH semantics.
3. proxy_filter.cc — defense in depth: catch std::exception in
   onData() alongside the existing ProtocolError catch, so any future
   decoder exception maps to the downstream protocol error path instead of
   aborting the process.

Testing

• The existing RESP codec tests pass with the refactored logic.
• Inputs like SET key "x\x41" (literal x before hex escape) no longer
  crash; they decode the hex escape correctly.
• Inputs with single hex digit (\x4) are left as-is (matching Redis
  behavior for incomplete escapes).
• OOB read at s[s.size() - 3] when s.size() == 2 (first hex digit on
  a fresh escape) is eliminated by the state tracking approach.

[repokitteh-read-only]

Hi @waterWang, welcome and thank you for your contribution.

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🐱

Caused by: #46643 was opened by waterWang.

see: more, trace.