fix(windows): configure OpenSSL roots and stabilize Bazel builds

[jinseopkim0]

Summary
This PR fixes build and test failures in the Kokoro Windows Bazel environment. It configures the build to correctly handle OpenSSL certificate roots for curl and temporarily excludes specific targets that are currently unstable on Windows.

kokoro/windows/bazel-release-2019 and kokoro/windows/bazel-release-2022 pass with this PR.

Changes

• Stabilize CI: Temporarily excluded specific Bigtable, Storage, and Pub/Sub targets that are causing CI failures on Windows (see Failing google/cloud/bigtable:internal_query_plan_test for kokoro windows bazel CI #15678).
• Update Examples: Modified the Storage quickstart and README documentation to support the CURL_CA_BUNDLE environment variable. This allows the examples to locate the custom CA bundle required in the hermetic build environment.
• Configure Roots: Updated bazel.ps1 to install root certificates and set the CURL_CA_BUNDLE environment variable during the test phase.

Reasoning
The Windows CI environment uses a hermetic build where standard system roots may not be sufficient or accessible. Configuring the client to use explicitly provided roots via environment variables resolves authentication failures during integration tests.

Note: #15822 has been created to investigate automatic detection of CA roots from environment variables (e.g. CURL_CA_BUNDLE)

[gemini-code-assist]

Summary of Changes

Hello @jinseopkim0, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses and resolves build and test failures occurring in the Kokoro Windows CI environment. It implements a robust mechanism for the C++ client to correctly locate and utilize CA certificates by integrating the CURL_CA_BUNDLE environment variable. Additionally, it temporarily excludes specific Bazel targets that are currently causing instability on Windows from the automated build and test processes, aiming to restore CI stability.

Highlights

• Windows CI CA Bundle Configuration: The C++ client initialization in quickstart examples and READMEs now conditionally uses the CURL_CA_BUNDLE environment variable to specify CA roots, addressing certificate issues in the Windows CI environment.
• Bazel Build/Test Exclusions: Several problematic Bazel targets, including specific Bigtable, Storage, and Pub/Sub samples/tests, are now explicitly excluded from the Windows Bazel build and test commands to prevent failures.
• Kokoro Environment Variable Setup: The Kokoro Windows build script (bazel.ps1) has been updated to set the CURL_CA_BUNDLE environment variable, ensuring the C++ client can locate the necessary CA certificates during integration tests.

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[gemini-code-assist]

Code Review

This pull request introduces fixes for the Kokoro Windows Bazel CI build. The changes include updating the C++ quickstart examples to respect the CURL_CA_BUNDLE environment variable for custom CA certificates, which is necessary for the hermetic build environment. The corresponding Bazel build script is updated to set this environment variable during integration tests. Additionally, several build targets that are failing on Windows are now excluded from the bazel build and bazel test commands to unblock the CI. The changes are correct and well-implemented. I have no specific comments.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.94%. Comparing base (826dd38) to head (e4b98e3).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15818      +/-   ##
==========================================
- Coverage   92.95%   92.94%   -0.01%     
==========================================
  Files        2458     2458              
  Lines      227589   227589              
==========================================
- Hits       211547   211537      -10     
- Misses      16042    16052      +10     

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