bug(gapic-node-processing): setOnlyDefaultSystemTests incorrectly matches substring on absolute path

[zhumin8]

Description

In gapic-node-processing/src/combine-libraries.ts, setOnlyDefaultSystemTests() filters out sample test fixtures from non-default versions when combining multi-version libraries (e.g., google-iam with v2 and v3).

It currently uses .includes(defaultVersion) on the full absolute file path:

function setOnlyDefaultSystemTests(defaultVersion: string, filePaths: FilePaths[]) {
  const systemTestRegex = new RegExp('system-test/fixtures/sample/src');
  for (let i = filePaths.length - 1; i >= 0; i--) {
    const filePathObj = filePaths[i];
    const normalizedPath = filePathObj.filePath.replace(/\\/g, '/');
    if (systemTestRegex.test(normalizedPath) &&
        !normalizedPath.includes(defaultVersion)) {
      filePaths.splice(i, 1);
    }
  }
}

Bug Behavior

If the workspace or temporary directory path where Librarian/generator runs happens to contain the substring defaultVersion (for example, /tmp/upgrade-nodejs-v2Mp8W containing "v2"), normalizedPath.includes("v2") evaluates to true for all API versions (including v3).

As a result:

1. !normalizedPath.includes(defaultVersion) evaluates to false.
2. v3 fixture files are not filtered out.
3. v3 sample fixtures overwrite v2 sample fixtures, generating unintended client diffs (e.g., packages/google-iam/system-test/fixtures/sample/src/index.ts exporting AccessPoliciesClient instead of PoliciesClient).

Reference PR & CI Breakage

This issue was observed during Librarian version upgrade in:

• Pull Request: chore: update librarian version to v0.44.0 #9340
• Affected Commit: 559da2d4 (where mktemp -d generated /tmp/upgrade-nodejs-v2Mp8W)
• CI Failure: https://github.com/googleapis/google-cloud-node/actions/runs/34998266744/job/104479939975?pr=9340 (clean CI runner without v2 in path did not generate the v3 diffs, causing a git diff discrepancy).

Suggested Fix

Match the version directory boundary specifically rather than performing an unconstrained substring check across the absolute path:

const versionDirRegex = new RegExp(`(^|/)${defaultVersion}(/|$)`);
if (systemTestRegex.test(normalizedPath) && !versionDirRegex.test(normalizedPath)) {
  filePaths.splice(i, 1);
}

[tejas5038]

I would like to work on this. I plan to make the version check path-segment-aware instead of matching the default version as an arbitrary substring of the absolute workspace path, and add focused regression coverage where the temporary parent path itself contains the version token. Please let me know if this approach is aligned before I open a PR.