chore: update proton and remove deprecated billing account org_id usage

[rohilsurana]

Problem

The protobuf schema deprecated org_id, billing_id, and project_id fields in several billing account API requests because the backend can infer these IDs from other parameters (e.g., org_id can be inferred from billing_account_id). However, the backend was still accepting and using these deprecated fields from client requests, creating a security risk where clients could potentially manipulate these IDs.

Solution

This PR removes all usage of deprecated fields and implements server-side inference of IDs for security:

1. Updated PROTON_COMMIT to latest version (b1687af7) with deprecation markers
2. Backend handlers no longer accept or use deprecated org_id from requests:
   • UpdateBillingAccount: Now infers org_id from billing account ID
   • UpdateBillingAccountDetails: Fetches customer to get org_id for audit log
   • billing/customer/service.go: Always uses existing customer's org_id instead of request
3. Authorization interceptors now infer org_id server-side for security:
   • Added getOrgForBillingAccount helper that fetches org_id from billing account
   • Updated all billing account interceptors to use inferred org_id for authorization checks
   • This prevents clients from bypassing authorization by providing incorrect org_id
4. Proto files regenerated with deprecation markers and updated API documentation

Security Impact

• ✅ Clients can no longer manipulate org_id to bypass authorization
• ✅ All org_id values used for authorization are now server-controlled
• ✅ Maintains backward compatibility (deprecated fields still accepted but ignored)

Test plan

• Unit tests pass
• Linting passes
• Build succeeds
• CI tests pass

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Project	Deployment	Review	Updated (UTC)
frontier	Ready	Preview, Comment	Jan 21, 2026 6:07am

[coveralls]

Pull Request Test Coverage Report for Build 21199174080

Details

• 13 of 57 (22.81%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.005%) to 37.882%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
internal/api/v1beta1connect/billing_customer.go	10	13	76.92%
internal/api/v1beta1connect/v1beta1connect.go	0	6	0.0%
pkg/server/connect_interceptors/authorization.go	0	35	0.0%

Totals
Change from base Build 21060277044:	-0.005%
Covered Lines:	15887
Relevant Lines:	41938

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💛 - Coveralls