(MOT-4407) fix(harness): report context usage truthfully

[rohitg00]

Refs MOT-4407

What

• make the Harness context chip provider-agnostic and explicit about where each number comes from
• separate provider-reported prompt totals from provider-tokenizer, local-tokenizer, or chars-per-four breakdowns
• show cache creation, cache hits, and reasoning only when the provider reports them
• derive session cost from the complete assistant transcript and report it as unavailable when any generation lacks pricing
• exclude the deterministic current assistant entry during redelivery so a retried step cannot be double-counted
• add focused presentation tests and update the metrics schema

Why

The context popover previously described mixed accounting as exact · tokenizer, rendered absent cache fields as zero, and could present a partial or missing price as a complete session total. GLM-5.3 made the mismatch visible, but the issue was in the shared Harness layer.

This applies to every current and future provider through the common Usage contract. Provider adapters remain responsible only for accurately parsing the usage fields their API returns.

Verification

• cargo fmt --manifest-path harness/Cargo.toml --all -- --check
• SKIP_UI_BUILD=1 cargo clippy --manifest-path harness/Cargo.toml -p harness --all-targets --all-features -- -D warnings
• SKIP_UI_BUILD=1 cargo test --manifest-path harness/Cargo.toml -p harness --all-features (329 tests)
• pnpm --dir harness/ui test (8 tests)
• pnpm --dir harness/ui build
• isolated Harness integration playground UI-001 scenario passed in Chromium; the popover showed provider usage, local breakdown, truthful cache/cost state, and no console warnings or errors
• git diff --check

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Configuration used: Organization UI

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Run ID: 6f4d0ab4-31a4-48b8-9bd4-ad4bb8e969ea

📥 Commits

Reviewing files that changed from the base of the PR and between 9d0df97 and 32efce9.

⛔ Files ignored due to path filters (1)

• pnpm-lock.yaml is excluded by !**/pnpm-lock.yaml

📒 Files selected for processing (8)

• harness/src/context_snapshot.rs
• harness/src/turn_loop.rs
• harness/tests/golden/schemas/harness.metrics.json
• harness/ui/package.json
• harness/ui/src/context-chip/index.tsx
• harness/ui/src/context-chip/presentation.test.ts
• harness/ui/src/context-chip/presentation.ts
• harness/ui/src/lib/metrics.ts

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skill-check — worker

0 verified, 59 skipped (no docs/).

Layer	Result
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vale	✓
ai	✓
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Four for four. Nicely done.