Port upstream 0.55.0: z.ai CN account balance

[Finesssee]

Summary

• port upstream CodexBar v0.55.0 BigModel CN account-balance support from Add BigModel CN account balance to z.ai provider steipete/CodexBar#3109
• perform the CN-only balance lookup against www.bigmodel.cn with a 5-second request bound
• surface the remaining balance as a provider detail row without turning prepaid balance into spend
• keep the lookup best-effort so quota display still succeeds if the balance endpoint fails

Upstream evidence

• target release: v0.55.0
• upstream commit: cdc456ae0 / Add BigModel CN account balance to z.ai provider steipete/CodexBar#3109

Validation

• C:/Users/mac/.cargo/bin/cargo.exe test -p codexbar zai -> 38 passed
• focused rustfmt on rust/src/providers/zai/mod.rs
• git diff --check -> pass

Scope

Isolated BigModel CN provider-detail port. Global z.ai behavior and spend accounting are unchanged.

[Finesssee]

Thermo-nuclear re-review

Previous finding is closed: CN balance request/parsing moved into balance.rs; zai/mod.rs is back below the thermo threshold at 983 lines. 38/38 focused tests and strict Clippy pass.

Verdict: APPROVED.