Add regression test for cdeps propagation through embed chains

This test verifies that cdeps are properly propagated when a go_library
with cdeps embeds another go_library with cgo=True, and a go_test embeds
the library with cdeps. This pattern was broken in v0.51.0 and fixed in
commit a494a68.

The test reuses existing native_dep cc_library to minimize test
complexity and follows the same pattern as the external reproducer.

Author: jsharpe

tests/core/cgo/BUILD.bazel

@@ -507,6 +507,7 @@ cc_library(
     hdrs = ["native_dep.h"],
     # Force static linking to ensure that the build doesn't succeed by
     # accidentally picking up the shared library in the search path.
+    alwayslink = True,
     linkstatic = True,
 )
 
@@ -556,3 +557,30 @@ go_bazel_test(
         "//conditions:default": ["@platforms//:incompatible"],
     }),
 )
+
+# Regression test for cdeps propagation through embed chains.
+# Tests that when a go_library with cdeps embeds another go_library with cgo=True,
+# and a go_test embeds the library with cdeps, the cdeps are properly propagated.
+# This was broken in rules_go v0.51.0+ and fixed in this commit.
+go_library(
+    name = "cdeps_embed_base",
+    srcs = [
+        "cdeps_embed_base.go",
+        "cdeps_embed.h",
+    ],
+    cgo = True,
+    importpath = "github.com/bazelbuild/rules_go/tests/core/cgo/cdeps_embed",
+)
+
+go_library(
+    name = "cdeps_embed_with_cdeps",
+    cdeps = [":native_dep"],
+    embed = [":cdeps_embed_base"],
+    importpath = "github.com/bazelbuild/rules_go/tests/core/cgo/cdeps_embed",
+)
+
+go_test(
+    name = "cdeps_embed_test",
+    srcs = ["cdeps_embed_test.go"],
+    embed = [":cdeps_embed_with_cdeps"],
+)

tests/core/cgo/cdeps_embed.h

@@ -0,0 +1,6 @@
+#ifndef CDEPS_EMBED_H
+#define CDEPS_EMBED_H
+
+void native_greeting(void);
+
+#endif

tests/core/cgo/cdeps_embed_base.go

@@ -0,0 +1,8 @@
+package cdeps_embed
+
+// #include "cdeps_embed.h"
+import "C"
+
+func CallNative() {
+	C.native_greeting()
+}

tests/core/cgo/cdeps_embed_test.go

@@ -0,0 +1,15 @@
+package cdeps_embed
+
+import "testing"
+
+// TestCdepsEmbedPropagation tests that cdeps are properly propagated
+// when a go_library with cdeps embeds another go_library with cgo=True,
+// and a go_test embeds the library with cdeps.
+//
+// This is a regression test for the issue where cdeps were not being
+// propagated through embed chains in rules_go v0.51.0+.
+func TestCdepsEmbedPropagation(t *testing.T) {
+	// If cdeps are properly propagated, this will link successfully
+	// and call the C function from native_dep.
+	CallNative()
+}