Enforce bounds checking in `structured_headers::Dictionary::operator[](size_t)`.

Add `QUICHE_CHECK_LT(idx, members_.size())` to both `Dictionary::operator[](size_t)` overloads (which `Dictionary::at(size_t)` also delegates to), aligning their behavior with their header documentation that both `operator[](size_t)` and `at(size_t)` abort the program on out-of-bounds access.

PiperOrigin-RevId: 1000942364

quiche/common/structured_headers.cc

diff --git a/quiche/common/structured_headers.cc b/quiche/common/structured_headers.cc
index 671a67e..d31ebee 100644
--- a/quiche/common/structured_headers.cc
+++ b/quiche/common/structured_headers.cc
@@ -1001,9 +1001,11 @@
 Dictionary::iterator Dictionary::end() { return members_.end(); }
 Dictionary::const_iterator Dictionary::end() const { return members_.end(); }
 ParameterizedMember& Dictionary::operator[](std::size_t idx) {
+  QUICHE_CHECK_LT(idx, members_.size());
   return members_[idx].second;
 }
 const ParameterizedMember& Dictionary::operator[](std::size_t idx) const {
+  QUICHE_CHECK_LT(idx, members_.size());
   return members_[idx].second;
 }
 ParameterizedMember& Dictionary::at(std::size_t idx) { return (*this)[idx]; }