Makes `SpdyAcceptChIR` single-argument constructor `explicit`.

Marks the `SpdyAcceptChIR(std::vector<AcceptChOriginValuePair>)` constructor as
`explicit` and explicitly initializes `AcceptChOriginValuePair` in
`spdy_protocol_test` to resolve ambiguous constructor overload resolution in
newer Clang versions (e.g., clang-22 on Linux).

PiperOrigin-RevId: 982552227

quiche/http2/core/spdy_protocol.h

diff --git a/quiche/http2/core/spdy_protocol.h b/quiche/http2/core/spdy_protocol.h
index d074786..c6a752f 100644
--- a/quiche/http2/core/spdy_protocol.h
+++ b/quiche/http2/core/spdy_protocol.h
@@ -1277,7 +1277,7 @@
 
 class QUICHE_EXPORT SpdyAcceptChIR : public SpdyFrameIR {
  public:
-  SpdyAcceptChIR(std::vector<AcceptChOriginValuePair> entries)
+  explicit SpdyAcceptChIR(std::vector<AcceptChOriginValuePair> entries)
       : entries_(std::move(entries)) {}
   SpdyAcceptChIR(const SpdyAcceptChIR&) = delete;
   SpdyAcceptChIR& operator=(const SpdyAcceptChIR&) = delete;

quiche/http2/core/spdy_protocol_test.cc

diff --git a/quiche/http2/core/spdy_protocol_test.cc b/quiche/http2/core/spdy_protocol_test.cc
index 82100e3..82328c7 100644
--- a/quiche/http2/core/spdy_protocol_test.cc
+++ b/quiche/http2/core/spdy_protocol_test.cc
@@ -261,7 +261,7 @@
   // short to trigger the move optimization, and instead a copy occurs.
   std::string baz = "the quick brown fox";
   SpdyDataIR d5(/* stream_id = */ 5, std::move(baz));
-  EXPECT_EQ("", baz);
+  EXPECT_EQ("", baz);  // NOLINT(bugprone-use-after-move)
   EXPECT_EQ(absl::string_view(d5.data(), d5.data_len()), "the quick brown fox");
 
   // Confirms makes a copy of string literal.
@@ -646,7 +646,8 @@
   EXPECT_TRUE(SerializeFrame(accept_ch, builder));
   SpdySerializedFrame serialized = builder.take();
 
-  SpdyAcceptChIR accept_ch_ir({{"example.com", "sec-ch-ua"}});
+  SpdyAcceptChIR accept_ch_ir(
+      {AcceptChOriginValuePair{.origin = "example.com", .value = "sec-ch-ua"}});
   SpdySerializedFrame expected = framer.SerializeAcceptCh(accept_ch_ir);
 
   EXPECT_EQ(absl::string_view(expected), absl::string_view(serialized));