feat(billing): Add has_remaining_capacity

[armenzg]

Summary

• Adds has_remaining_capacity on LineItemUsageSummary (field 8) and a pool-level mirror on SharedLineItemUsageSummary (field 5) so quota enforcement can detect remaining Engagement unit grant capacity without changing reserved_budget_exhausted / payg_budget_exhausted semantics.

Follow-up: getsentry bumps this package and uses both fields for UsagePricer + ContractBudgetEnforcer + get_unit_grants windows (https://github.com/getsentry/getsentry/pull/21233).

Test plan

• Buf lint / format / breaking checks
• getsentry consumes regenerated bindings and exercises mid-period grant windows

[github-actions]

The latest Buf updates on your PR. Results from workflow ci / buf-checks (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Jul 30, 2026, 1:02 PM

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

Bugbot Autofix prepared a fix for the issue found in the latest run.

• ✅ Fixed: Rust proto bindings out of sync
  • Removed the date_added field (tag 4) from RecurringCredit Rust bindings to match the proto definition which only defines fields 1-3.

Or push these changes by commenting:

@cursor push 415b80a110

Preview (415b80a110)

diff --git a/rust/src/sentry_protos.billing.v1.services.engagement.v1.rs b/rust/src/sentry_protos.billing.v1.services.engagement.v1.rs
--- a/rust/src/sentry_protos.billing.v1.services.engagement.v1.rs
+++ b/rust/src/sentry_protos.billing.v1.services.engagement.v1.rs
@@ -202,11 +202,6 @@
     pub r#type: i32,
     #[prost(uint64, tag = "3")]
     pub amount: u64,
-    /// UTC calendar day the credit row was created. Unit-grant consumers use
-    /// max(ondemand_period_start, date_added) so mid-period grants only drain
-    /// against usage on/after the grant day (forward headroom).
-    #[prost(message, optional, tag = "4")]
-    pub date_added: ::core::option::Option<super::super::super::Date>,
 }
 /// Denomination of a RecurringCredit.
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, PartialOrd, Ord, ::prost::Enumeration)]

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