fix: header incompatibility for gRPC

[G5277]

🔁 Pull Request Template – BharatMLStack

📌 Summary

Updated gRPC server interceptors to use lowercase headers (online_feature_store_caller_id, online_feature_store_auth_token), ensuring HTTP/2 compliance and proper authentication with the Python client.

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📂 Modules Affected

• horizon (Real-time systems / networking)
• online-feature-store (Feature serving infra)
• trufflebox-ui (Admin panel / UI)
• infra (Docker, CI/CD, GCP/AWS setup)
• docs (Documentation updates)
• Other: ___________

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✅ Type of Change

• Feature addition
• Bug fix
• Infra / build system change
• Performance improvement
• Refactor
• Documentation
• Other: ___________

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📊 Benchmark / Metrics (if applicable)

N/A — change only updates header constants; no performance impact.

[turbo-turtle-github]

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