Convert proto toolchain string to Label

Without it the still slightly weird setup of Bazel unit tests causes problems.

PiperOrigin-RevId: 684391633

bazel/private/bazel_cc_proto_library.bzl

@@ -13,7 +13,7 @@ load("//bazel/common:proto_info.bzl", "ProtoInfo")
 load("//bazel/private:cc_proto_support.bzl", "cc_proto_compile_and_link")
 load("//bazel/private:toolchain_helpers.bzl", "toolchains")
 
-_CC_PROTO_TOOLCHAIN = "//bazel/private:cc_toolchain_type"
+_CC_PROTO_TOOLCHAIN = Label("//bazel/private:cc_toolchain_type")
 
 _ProtoCcFilesInfo = provider(fields = ["files"], doc = "Provide cc proto files.")
 _ProtoCcHeaderInfo = provider(fields = ["headers"], doc = "Provide cc proto headers.")

bazel/private/bazel_java_proto_library_rule.bzl

@@ -12,7 +12,7 @@ load("//bazel/common:proto_info.bzl", "ProtoInfo")
 load("//bazel/private:java_proto_support.bzl", "JavaProtoAspectInfo", "java_compile_for_protos", "java_info_merge_for_protos")
 load("//bazel/private:toolchain_helpers.bzl", "toolchains")
 
-_JAVA_PROTO_TOOLCHAIN = "//bazel/private:java_toolchain_type"
+_JAVA_PROTO_TOOLCHAIN = Label("//bazel/private:java_toolchain_type")
 
 def _filter_provider(provider, *attrs):
     return [dep[provider] for attr in attrs for dep in attr if provider in dep]

bazel/private/java_lite_proto_library.bzl

@@ -16,7 +16,7 @@ load("//bazel/private:toolchain_helpers.bzl", "toolchains")
 
 _PROTO_TOOLCHAIN_ATTR = "_aspect_proto_toolchain_for_javalite"
 
-_JAVA_LITE_PROTO_TOOLCHAIN = "//bazel/private:javalite_toolchain_type"
+_JAVA_LITE_PROTO_TOOLCHAIN = Label("//bazel/private:javalite_toolchain_type")
 
 def _aspect_impl(target, ctx):
     """Generates and compiles Java code for a proto_library dependency graph.

bazel/private/toolchain_helpers.bzl

@@ -45,5 +45,5 @@ toolchains = struct(
     find_toolchain = _find_toolchain,
     if_legacy_toolchain = _if_legacy_toolchain,
     INCOMPATIBLE_ENABLE_PROTO_TOOLCHAIN_RESOLUTION = _incompatible_toolchain_resolution,
-    PROTO_TOOLCHAIN = "//bazel/private:proto_toolchain_type",
+    PROTO_TOOLCHAIN = Label("//bazel/private:proto_toolchain_type"),
 )