[CTORNDGAIN-1178] Add script existence checks before chmod in CI workflows

[github-actions]

Jira: https://griddynamics.atlassian.net/browse/CTORNDGAIN-1178

Summary

• Adds [ -f <script> ] || { echo "::error::..."; exit 1; } guard before every chmod +x in CI workflows
• Affects publish-ims-mcp.yml (2 chmod calls) and validate-test-cases.yml (1 chmod call)
• Guards produce a clear GitHub Actions error annotation and a non-zero exit when the script is missing, instead of silently running chmod on a nonexistent path

Testing notes

• All three target scripts (validate-types.sh, ims-mcp-server/build.sh, .github/scripts/run-test-cases.sh) are already committed with mode 100755, so the guards will not affect normal CI runs
• The guard pattern [ -f <file> ] || { echo "::error::..."; exit 1; } is POSIX sh-compatible and works on ubuntu-latest

Assumptions

None — script paths are already tracked in git and existence is deterministic.

CI Workflow Changes (Manual)

⚠️ These changes must be applied manually to .github/workflows/ because the CI token cannot push workflow files.

.github/workflows/publish-ims-mcp.yml

     - name: Run type validation
       run: |
+        [ -f validate-types.sh ] || { echo "::error::validate-types.sh not found"; exit 1; }
         chmod +x validate-types.sh
         ./validate-types.sh

     - name: Build package
       working-directory: ./ims-mcp-server
       run: |
         # V2: bootstrap.md no longer bundled (loaded from Rosetta Server at startup)
+        [ -f build.sh ] || { echo "::error::build.sh not found"; exit 1; }
         chmod +x build.sh
         ./build.sh

.github/workflows/validate-test-cases.yml

         run: |
           echo "Running test cases for changed prompts..."
+          [ -f .github/scripts/run-test-cases.sh ] || { echo "::error::.github/scripts/run-test-cases.sh not found"; exit 1; }
           chmod +x .github/scripts/run-test-cases.sh
           .github/scripts/run-test-cases.sh

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