fix(py-sdk): lowercase gRPC metadata header keys

[Mrsandeep27]

Summary

Fixes #165 — `GRPCFeatureClient` cannot send a single request from Python because grpcio rejects the uppercase metadata keys at client-side validation.

Problem

`py-sdk/grpc_feature_client/grpc_feature_client/client.py` declares:
```python
HEADER_CALLER_ID = "ONLINE-FEATURE-STORE-CALLER-ID"
HEADER_CALLER_TOKEN = "ONLINE-FEATURE-STORE-AUTH-TOKEN"
```

When these are passed to `.with_call(metadata=...)`, grpcio's `validate_metadata` checks each key against HTTP/2's grammar — ASCII-lowercase only — and fails with:

```
INTERNAL: Illegal header key
```

The call never leaves the Python process.

Fix

Rename both constants to lowercase:
```python
HEADER_CALLER_ID = "online-feature-store-caller-id"
HEADER_CALLER_TOKEN = "online-feature-store-auth-token"
```

Server compatibility

The server reads these via `metadata.FromIncomingContext(ctx)` + `md.Get(key)` (go-sdk/pkg/api/request_context.go:42-117). `metadata.MD.Get` is case-insensitive (grpc-go lowercases on lookup), so the server accepts both cases. The existing Go SDK (`go-sdk/pkg/onfs/client.go:18-19`) still declares uppercase constants — grpc-go silently normalizes them to lowercase on the wire, so there's no wire-level divergence between the two clients.

No changes needed on the Go side. The Go constants can be lowercased in a separate cleanup PR if you want consistency, but it isn't required for this fix.

Test plan

• Python SDK builds a unary call with valid metadata (no more `Illegal header key`).
• Server-side auth middleware still resolves the caller ID / token.
• Existing Go SDK continues to work unchanged (headers are lowercased by grpc-go on send).

Summary by CodeRabbit

• Bug Fixes
  • Updated gRPC authentication metadata headers to use lowercase naming conventions, ensuring proper communication and compatibility with gRPC services.

[turbo-turtle-github]

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[turbo-turtle-github]

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 8ac9e31 and ec4d559.

📒 Files selected for processing (1)

• py-sdk/grpc_feature_client/grpc_feature_client/client.py

---

Walkthrough

The PR updates gRPC metadata header keys in the Python client from uppercase to lowercase format. Constants HEADER_CALLER_ID and HEADER_CALLER_TOKEN are changed from "ONLINE-FEATURE-STORE-CALLER-ID" and "ONLINE-FEATURE-STORE-AUTH-TOKEN" to their lowercase equivalents.

Changes

Cohort / File(s)	Summary
gRPC Metadata Header Keys py-sdk/grpc_feature_client/grpc_feature_client/client.py	Updated internal constants used for gRPC metadata header keys from uppercase to lowercase format to comply with Python gRPC requirements.

Assessment against linked issues

Objective	Addressed	Explanation
Convert header keys to lowercase to fix gRPC Python compatibility error [#165]	❓	PR changes header keys to lowercase; however, the linked issue references HTTP/2 restrictions requiring "lowercase ASCII without hyphens," but the converted headers still contain hyphens. Clarification needed on whether hyphens in lowercase keys are acceptable.

Suggested labels

coderabbit-bugfix

🚥 Pre-merge checks | ✅ 1

✅ Passed checks (1 passed)

Check name	Status	Explanation
Dynamic Configuration Validation	✅ Passed	No application-dyn-*.yml files were modified in this PR.

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