Setup CodSpeed for continuous benchmarking

[codspeed-hq]

Summary

This PR sets up CodSpeed for continuous performance monitoring on the pyhdb-rs repository:

• Replaced criterion with codspeed-criterion-compat v4.3.0 as a drop-in replacement
• Created .github/workflows/codspeed.yml for automated benchmark execution
• Added CodSpeed badge to README.md
• Fixed test compilation by feature-gating tests that require the test-utils feature
• Excluded Python benchmarks (hdbconnect-py) from the CodSpeed workflow to avoid linker issues

CodSpeed Integration

CodSpeed provides accurate, reproducible performance measurements in CI by using specialized instrumentation that eliminates environmental noise. The integration uses:

• Mode: Simulation (for consistent, hardware-agnostic measurements)
• Authentication: OIDC-based (secure, no token needed)
• Benchmark Framework: criterion (via codspeed-criterion-compat)
• Scope: Rust benchmarks only (hdbconnect-arrow and hdbconnect-mcp)

Changes

1. Cargo.toml: Updated criterion dependency to use codspeed-criterion-compat
2. Cargo.lock: Updated lockfile with CodSpeed dependencies
3. README.md: Added CodSpeed badge
4. .github/workflows/codspeed.yml: New workflow for running benchmarks (excludes Python crate)
5. processor.rs: Added #[cfg(feature = "test-utils")] to tests using decimal_str()

Fixes

• Test Compilation: Added feature gates to tests that use methods requiring test-utils feature
• Linker Issues: Excluded hdbconnect-py crate which has Python runtime dependencies incompatible with CodSpeed's static build approach

Test Plan

• Verify benchmarks compile successfully with cargo codspeed build --workspace --exclude hdbconnect-py
• Verify benchmarks run with cargo codspeed run --workspace --exclude hdbconnect-py
• Verify CI workflow completes successfully
• Check that CodSpeed reports are generated and accessible

Next Steps

Once this PR is merged:

1. CodSpeed will automatically track performance on every commit to main
2. Pull requests will receive performance comparison reports
3. Visit https://codspeed.io/bug-ops/pyhdb-rs to view detailed performance trends

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@           Coverage Diff           @@
##             main      #85   +/-   ##
=======================================
  Coverage   84.09%   84.09%           
=======================================
  Files          63       63           
  Lines       12618    12618           
=======================================
  Hits        10611    10611           
  Misses       2007     2007           

Flag	Coverage Δ
hdbconnect-arrow	84.03% <ø> (ø)
hdbconnect-mcp	84.03% <ø> (ø)
python	100.00% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...rates/hdbconnect-arrow/src/conversion/processor.rs	97.49% <ø> (ø)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[codspeed-hq]

Merging this PR will improve performance by 10.75%

⚡ 1 improved benchmark
✅ 136 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	get_no_ttl_check	6.6 µs	6 µs	+10.75%

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_{Comparing setup-codspeed-1770038502668 (ddbfe5b) with main (56cc23d)}