Port upstream 0.70.0: price Mistral usage by event type, API zone and service tier

[Finesssee]

Summary

Ports upstream v0.70.0 steipete#4076: Mistral API spend now prices each billed usage row by its event type, API zone and service tier, not by billing metric and group alone.

• The bug: the billing price table lists one metric several times, once per API zone and service tier and again under other event types. Keyed by metric::group, the last row won. In upstream's real September 2026 sample, mistral-medium-3-5 input was priced at the per-second audio rate, which is 100x the token rate, and cached and output tokens took the EU priority rates.
• The fix: usage rows and prices now decode event_type, api_zone and service_tier. A row takes the price with the same event type, metric, group, zone and tier.
• Legacy tables: a price row that omits both zone and tier is the only fallback, for rows with the same event type, metric and group. A price that names only one of them, or a different zone or tier, never prices the row, so no zone or tier is guessed. Tables and rows that omit every dimension still match as before.
• The lookup lives in a new providers/mistral/pricing.rs submodule, which keeps mod.rs at 853 lines.

Token totals are unchanged: they already count consumed units per category.

Stacked on #657 (port/micro-0.68.0-mistral-plan-detail), like #717. This PR and #717 change different parts of providers/mistral/mod.rs.

Upstream reference

• fix(mistral): price billing usage by event type, zone, and tier steipete/CodexBar#4076, upstream b468e0dbf3 (v0.70.0 release notes: "Mistral: price billing usage by event type, API zone, and service tier").
• Tag-pinned v0.70.0 sources: MistralModels.swift (apiZone and serviceTier on MistralUsageEntry and MistralPrice), MistralUsageFetcher.swift (PriceKey, the exact key then the zoneless key, a later duplicate wins), Tests/CodexBarTests/MistralUsageParserTests.swift and docs/mistral.md.

Ported / Deferred

Change	Status
event_type, api_zone and service_tier on usage rows and prices	Ported
Exact price key, then the unqualified legacy row; unmatched rows cost 0	Ported
Upstream tests: price by event type, zone and tier in both table orders; legacy prices without guessing a zone or tier	Ported (pricing.rs tests)
Category table refactor in MistralUsageFetcher (library tokens count only in daily buckets)	Not needed. It is a pure refactor upstream; Windows already aggregates the same categories and has no Mistral daily buckets.
Daily bucket and 30-day history cost	Not applicable. The Windows Mistral provider reports the monthly cost only.
docs/mistral.md paragraph	Not applicable. There is no Windows Mistral doc page; the rule is documented in the pricing.rs module comment.

Validation

Run in the worker worktree on Rust 1.98.0 at 12c366e7:

Command	Result
cargo +1.98.0 fmt --all --check	pass
cargo +1.98.0 clippy --workspace --all-targets -- -D warnings	pass
cargo +1.98.0 test -p codexbar providers::mistral	23 passed
cargo +1.98.0 test -p codexbar	2175 passed, 0 failed, 1 ignored
cargo +1.98.0 test -p codexbar-desktop-tauri -- --skip bootstrap_payload_exposes_every_provider_variant	463 passed, 0 failed

The skipped desktop test is the non-hermetic #684 test, which is not on this stacked base. #711 fixes it. The frontend is unchanged.

New tests in providers/mistral/pricing.rs:

• prices_entries_by_event_type_zone_and_tier_instead_of_the_last_matching_metric: upstream's September 2026 sample, in both table orders. The old metric::group key fails the original order.
• legacy_prices_match_qualified_usage_without_guessing_a_zone_or_tier: the unqualified price costs 40 billed units x 0.25 = 10. The EU-standard and global-priority prices cost 0, where the old key charged 10.
• exact_dimensions_win_over_the_unqualified_legacy_row, a_partly_qualified_price_is_not_a_fallback, event_type_must_match_even_for_the_legacy_row and rows_without_a_metric_or_group_have_no_price.

Affected areas

• rust/src/providers/mistral/pricing.rs (new): MistralPrice, PriceIndex and tests.
• rust/src/providers/mistral/mod.rs: usage-row fields and the price lookup.

UI proof

Not applicable: backend only. The change alters the Mistral spend amount, not any UI surface, and the tests above cover it.

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