Shard a few large OSS shell tests

Before:
```
//src/test/shell/bazel:bazel_java_test                          (cached) PASSED in 746.9s
//src/test/shell/bazel:bazel_java_test_jdk11_toolchain_head     (cached) PASSED in 307.3s
//src/test/shell/bazel:bazel_java_test_jdk17_toolchain_head     (cached) PASSED in 370.7s
//src/test/shell/bazel:bazel_java_test_jdk21_toolchain_head     (cached) PASSED in 340.6s
//src/test/shell/bazel:bazel_proto_library_test                 (cached) PASSED in 709.5s

```

After:
```
//src/test/shell/bazel:bazel_java_test                                   PASSED in 340.3s
//src/test/shell/bazel:bazel_java_test_jdk11_toolchain_head              PASSED in 221.2s
//src/test/shell/bazel:bazel_java_test_jdk17_toolchain_head              PASSED in 218.0s
//src/test/shell/bazel:bazel_java_test_jdk21_toolchain_head              PASSED in 315.0s
//src/test/shell/bazel:bazel_proto_library_test                          PASSED in 312.3s
```

Related: #23726
PiperOrigin-RevId: 678209188
Change-Id: I3b51584ba2893fe01a7ac084c99961623f0e4b02

src/test/shell/bazel/BUILD

@@ -272,6 +272,7 @@ sh_test(
         "@bazel_tools//tools/bash/runfiles",
     ],
     exec_compatible_with = ["//:highcpu_machine"],
+    shard_count = 2,
     tags = [
         "requires-network",  # For Bzlmod
     ],
@@ -304,6 +305,7 @@ JAVA_VERSIONS_COVERAGE = ("11", "17", "21")
             "@bazel_tools//tools/bash/runfiles",
         ],
         exec_compatible_with = ["//:highcpu_machine"],
+        shard_count = 2,
         tags = [
             "requires-network",  # For Bzlmod
         ],
@@ -384,6 +386,7 @@ sh_test(
     srcs = ["bazel_proto_library_test.sh"],
     data = [":test-deps"],
     exec_compatible_with = ["//:highcpu_machine"],
+    shard_count = 2,
     tags = [
         "no_windows",  # Doesn't work on Windows for unknown reason
         "requires-network",  # Allow this test to access internet to fetch rules_proto dependencies.