refactor: deprecate org_id in billing account CRUD RPCs

[rohilsurana]

Summary

• Deprecate org_id parameter in 7 billing account CRUD operations
• Update deprecation comments to correctly indicate org_id will be inferred from billing account id
• Follows the same pattern as PRs refactor(frontier): deprecate redundant request parameters from subscription RPC #435-refactor: deprecate billing_id and project_id in entitlement and usag… #438 for billing simplification

Changes

Deprecated org_id in:

1. GetBillingAccount
2. UpdateBillingAccount
3. RegisterBillingAccount
4. DeleteBillingAccount
5. EnableBillingAccount
6. DisableBillingAccount
7. GetBillingBalance
8. HasTrialed (both id and org_id deprecated, inferred from plan_id)

Not deprecated (correctly kept as required):

• CreateBillingAccount - needs org_id to create billing account
• ListBillingAccounts - needs org_id to filter accounts

Migration Path

Following the established pattern:

• org_id marked as deprecated in proto
• Backend will infer org_id from billing account id
• Existing paths remain for backward compatibility
• Will be removed in 2 future releases