chore: bump C++ server SDK to 3.10.1 in examples

[devin-ai-integration]

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Related issues

N/A

Describe the solution you've provided

Bumps the C++ Server-Side SDK dependency from 3.9.0 to 3.10.1 in the example Dockerfiles and the CI version variable.

Files changed:

• examples/hello-debian/Dockerfile
• examples/hello-haproxy/Dockerfile
• examples/hello-nginx/Dockerfile
• .github/variables/cpp-sdk-versions.env

Note: The README minimum version table was intentionally left at 3.9.0, since the SDK is compatible with both 3.9.x and 3.10.x and the column represents the minimum supported version.

CI coverage

All three examples are built and verified end-to-end in CI:

• plain-lua-examples (hello-debian) — builds from source with 3.10.1 and verifies output
• web-server-examples (hello-haproxy) — downloads 3.10.1 release artifacts, builds image, runs container, and verifies output
• web-server-examples (hello-nginx) — builds from source on pinned OpenResty, runs container, and verifies output

Human review checklist

• Verify the 3.10.1 release notes don't introduce behavioral changes beyond what the example smoke tests cover

Describe alternatives you've considered

N/A

Additional context

Link to Devin run: https://app.devin.ai/sessions/4ce7c6ebb2914ee580f50e0a840ebb9e
Requested by: rlamb@launchdarkly.com

[devin-ai-integration]

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