Shard Windows MSVC Bazel CI jobs into two shards.

Add an optional `shards` matrix dimension to the reusable Bazel workflow that uses `bazel query` and `--target_pattern_file` to partition test targets across shards without manually curating package lists, and split the Windows MSVC 2022 Bazel CI configurations into two shards to halve cold-cache test compilation and execution time.

PiperOrigin-RevId: 1007538480

.github/workflows/bazel.yml

diff --git a/.github/workflows/bazel.yml b/.github/workflows/bazel.yml
index 218cd0c..9aaa855 100644
--- a/.github/workflows/bazel.yml
+++ b/.github/workflows/bazel.yml
@@ -50,6 +50,11 @@
         required: false
         default: '["opt", "fastbuild"]'
         type: string
+      shards:
+        description: "JSON array of shards to split tests across (e.g. '[\"1/2\", \"2/2\"]')"
+        required: false
+        default: '[""]'
+        type: string
 
 permissions:
   contents: read
@@ -57,7 +62,7 @@
 jobs:
   test:
     runs-on: ${{ inputs.os }}
-    name: (${{ matrix.std }}, ${{ matrix.compilation_mode }})
+    name: (${{ matrix.std }}, ${{ matrix.compilation_mode }}${{ matrix.shard && format(', {0}', matrix.shard) || '' }})
     container:
       image: ${{ inputs.container_image }}
       options: --cap-add=SYS_PTRACE
@@ -70,6 +75,7 @@
       matrix:
         std: ${{ fromJSON(inputs.stds) }}
         compilation_mode: ${{ fromJSON(inputs.compilation_modes) }}
+        shard: ${{ fromJSON(inputs.shards) }}
     defaults:
       run:
         shell: bash
@@ -98,14 +104,21 @@
           external-cache: true
           repository-cache: true
           google-credentials: ${{ secrets.GAR_SERVICE_ACCOUNT }}
-          bazelrc: common --config=${{ inputs.remote_config }}
+          bazelrc: build --config=${{ inputs.remote_config }}
 
       - name: Fetch all external dependencies
         run: bazel fetch //...
 
+      - name: Shard test targets
+        if: ${{ matrix.shard != '' }}
+        run: |
+          IFS=/ read -r shard total <<< "${{ matrix.shard }}"
+          bazel query 'tests(//...)' | tr -d '\r' | awk "NR % $total == $shard - 1" > targets.txt
+
       - name: Run tests
         run: >
-          bazel test ...
+          bazel test
+          ${{ matrix.shard && '--target_pattern_file=targets.txt' || '...' }}
           --config=${{ inputs.toolchain }}
           --cxxopt=${{ startsWith(inputs.os, 'windows') && '/std:' || '-std=' }}${{ matrix.std }}
           --host_cxxopt=${{ startsWith(inputs.os, 'windows') && '/std:' || '-std=' }}${{ matrix.std }}

.github/workflows/ci.yml

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index b8a9423..4a9b6b5 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -157,11 +157,13 @@
           # windows-msvc-2022-cxx{17,20}-{dbg,opt}-bazel.cfg
           - name: MSVC 2022
             toolchain: ci-windows-msvc
+            shards: '["1/2", "2/2"]'
           # windows-msvc-2022-alternate-options-cxx{17,20}-dbg-bazel.cfg (dbg only)
           - name: MSVC 2022 Alternate Options
             toolchain: ci-windows-msvc
             alternate_options: true
             compilation_modes: '["dbg"]'
+            shards: '["1/2", "2/2"]'
           # windows-clangcl-cxx17-fastbuild-bazel.cfg
           - name: ClangCL
             toolchain: windows-clangcl
@@ -176,6 +178,7 @@
       alternate_options: ${{ matrix.config.alternate_options || false }}
       stds: ${{ matrix.config.stds || '["c++17", "c++20"]' }}
       compilation_modes: ${{ matrix.config.compilation_modes || '["dbg", "opt"]' }}
+      shards: ${{ matrix.config.shards || '[""]' }}
 
   macos:
     name: macOS ${{ matrix.config.name }} Bazel