perf(ci): replace mvn help:evaluate with native bash and sed extraction

[lqiu96]

This PR is part 2 of 2 in a stacked series:

1. fix: avoid running split ITs unexpectedly on unrelated changes (#12029) #14217: fix: avoid running split ITs unexpectedly on unrelated changes (#12029)
2. This PR: perf(ci): replace mvn help:evaluate with native bash and sed extraction

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Problem

In Kokoro CI scripts, mvn help:evaluate was used to parse POM values:

• In generate_modified_modules_list (.kokoro/common.sh), evaluating project.modules launched a full JVM and evaluated the monorepo POMs, taking 20–30+ seconds on every single CI run.
• In downstream-build.sh (.kokoro/presubmit/downstream-build.sh), evaluating gapic-showcase.version launched another Maven JVM process.

Changes

• Pure-Bash Module Extraction: Updated generate_modified_modules_list to use extract_pom_modules pom.xml, extracting all 270 modules in ~0.02s without JVM boot overhead.
• Sed Showcase Version Extraction: Updated downstream-build.sh to parse <gapic-showcase.version> directly using sed with fail-fast validation.
• Unit Tests: Added automated unit tests in .kokoro/common_test.sh for extract_pom_modules and generate_modified_modules_list.
• Refactoring & Best Practices: Scoped all temporary variables as local, used read -r module <<< "${module}" for pure-bash whitespace trimming, and anchored paths with ${scriptDir}/...

[gemini-code-assist]

Code Review

This pull request optimizes the Kokoro CI build scripts by replacing slow Maven commands with pure Bash alternatives to extract module lists and showcase versions, significantly reducing build setup times. It also introduces a helper function to skip integration and GraalVM tests for unmodified modules, backed by new unit tests. The code review feedback points out two important issues in the common script: first, a potential 'dubious ownership' Git error in Docker environments because the safe directory configuration is bypassed in some jobs; and second, a risk of script termination under 'set -e' when using a here-string read to trim whitespace. Both issues should be addressed using the provided code suggestions.

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.kokoro/common.sh (350-354)

Since generate_modified_modules_list is no longer called in integration-single and graalvm-single jobs, is_module_modified will be executed without git config --global --add safe.directory having been run. In Kokoro/Docker environments where the build runs under a different user, this will cause git diff to fail with a 'dubious ownership' error.

We should ensure git config --global --add safe.directory "$(realpath .)" is run before executing git diff inside is_module_modified as well.

  if [[ -z "${modified_files:-}" ]]; then
    git config --global --add safe.directory "$(realpath .)"
    local target_branch="${KOKORO_GITHUB_PULL_REQUEST_TARGET_BRANCH:-origin/main}"
    local target_commit="${KOKORO_GITHUB_PULL_REQUEST_COMMIT:-HEAD}"
    modified_files=$(git diff --name-only "${target_branch}...${target_commit}")
  fi

.kokoro/common.sh (125-126)

Using read -r module <<< "${module}" for trimming whitespace can be risky if set -e is enabled in the calling script. If module is empty or contains only whitespace, read will return a non-zero exit status (1), causing the script to abort immediately.

Additionally, here-strings (<<<) can be less efficient as they may write to temporary files or spawn subshells depending on the Bash version. Reverting to the original pure parameter expansion is safer, faster, and avoids any exit status issues.

      # Trim leading/trailing whitespace without spawning external processes
      module="${module#"${module%%[![:space:]]*}"}"
      module="${module%"${module##*[![:space:]]}"}"