Fix CI pipeline exit code masking with set -o pipefail

The benchmark command piped to tee was masking the exit code:
  cargo bench ... | tee out.txt

When cargo bench failed with exit code 101, tee succeeded with exit
code 0, making the whole pipeline appear successful. This caused:
- False success status on CI runs where benchmarks actually failed
- Subsequent steps running despite benchmark failures
- Broken benchmark results being committed to main

Solution: Add 'set -o pipefail' to ensure the pipeline returns the
exit code of the first failing command, making CI properly fail when
benchmarks fail.

Fixes part of #5

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: konard

.github/workflows/rust.yml

@@ -57,7 +57,9 @@ jobs:
           NEO4J_URI: bolt://localhost:7687
           NEO4J_USER: neo4j
           NEO4J_PASSWORD: password
-        run: cargo bench --bench bench -- --output-format bencher | tee out.txt
+        run: |
+          set -o pipefail
+          cargo bench --bench bench -- --output-format bencher | tee out.txt
 
       - name: Prepare benchmark results
         run: |