feat(usage): persist resolvedModel on provider_usage events

[FabioLeitao]

Summary

• Decode provider-echoed model from OpenAI-compatible streaming chunks into Usage.ResolvedModel, merge on collect (last non-empty wins), and persist as resolvedModel on provider_usage events.
• Does not change pricing model (registry.Require / CalculateCost unchanged); observability only (omitempty).
• Codex /responses path sets ResolvedModel from response.model when present.
• Merges upstream/main at 99721c76 (OpenCode session headers fix(providers): send stable session headers to OpenCode Twigpine/zero#1060) alongside this change.

Motivation

Routed providers (e.g. openrouter/free) need per-prompt attribution of the concrete model that served the request (herding-vicuna measurement). PATCHSPEC: PATCHSPEC_zero_resolved_model_attribution_2026-09-30 (operator vault).

Test plan

• go test ./internal/providers/openai ./internal/usage ./internal/zeroruntime (incl. new unit tests)
• make lint
• make vulncheck — no vulnerabilities
• Local go build -o zero-patched ./cmd/zero

Pre-existing / environment (not blocking this PR)

TestResolveReportsExplicitMaxTurns (internal/config) fails identically on isolated bases without this patch:

• a4a83049, 95c8fc14, 9e609c52 (feat(exec): default headless runs to the turn-budget ceiling Twigpine/zero#1049), 99721c76 (fix(providers): send stable session headers to OpenCode Twigpine/zero#1060), and this branch — same error: no active provider configured: active provider "" not found on subcase built-in default only.
• Not introduced by resolvedModel or merge fix(providers): send stable session headers to OpenCode Twigpine/zero#1060; no edits to that test.

Local Lynis umask 0007: permission-sensitive tests need ( umask 0022; make test ); with 0022, only config (above) + tui TestRelativeAgeFormatsLastActivity remain red (also unrelated to this diff).

make lint-static: 4 staticcheck QF* on main (unchanged). gosec ./...: repo baseline exit 1 (331 issues); none in touched paths.

Upstream (GBCU)

Generic for any router echoing model — propose to Gitlawb/zero via Discussions after merge here (no PR to upstream in this change).

[github-actions]

Zero automated PR review

Verdict: No blockers found

Blockers

• None found.

Validation

• [pass] Diff hygiene: git diff --check
• [pass] Tests: go test ./...
• [pass] Build: go run ./cmd/zero-release build
• [pass] Smoke build: go run ./cmd/zero-release smoke

Scope

Head: 063b75f4ecf2
Changed files (15): internal/providers/anthropic/provider.go, internal/providers/factory.go, internal/providers/factory_test.go, internal/providers/openai/codex_responses.go, internal/providers/openai/provider.go, internal/providers/openai/provider_test.go, internal/providers/openai/tool_state.go, internal/providers/openai/types.go, internal/providers/providerio/headers.go, internal/usage/report.go, internal/usage/report_test.go, internal/usage/token_economy_test.go, and 3 more

This deterministic review checks validation status and basic diff hygiene. A human reviewer still owns product judgment and design quality.