refactor(frontier): deprecate redundant request parameters from subscription RPC

[whoAbhishekSah]

Summary

Deprecate billing_id and org_id parameters in Frontier subscription RPCs while maintaining full backward compatibility.

Changes

• Mark billing_id as deprecated in all 5 subscription request messages (List, Get, Update, Cancel, Change)
• Mark org_id as deprecated in Get, Update, Cancel, and Change subscription requests
• Keep org_id in ListSubscriptions as the primary filter parameter
• Make deprecated fields optional with ignore_empty: true

Backward Compatibility

✅ No breaking changes - All existing clients continue to work:

• Deprecated fields remain in the proto schema
• Validation rules use ignore_empty: true to allow gradual migration
• Backend ignores deprecated parameters and infers values instead

Rationale

Each organization has exactly one billing account. The backend can infer:

• billing_id from org_id (for ListSubscriptions)
• Both billing_id and org_id from subscription_id (for Get/Update/Cancel/Change)

This simplifies the API and improves security by preventing clients from passing arbitrary billing IDs.

Related

Backend implementation: Frontier PR link