fix(provider): preserve terminal failure outcomes

[drewstone]

Summary

• Preserve explicit provider failures and nested provider error codes.
• Keep incomplete token receipts unknown without losing reported floors.
• Do not treat a recoverable child task failure as a failed turn.

Verification

• pnpm exec vitest run src/runtime/environment-provider.test.ts — 61 passed
• pnpm test — 3,577 passed, 9 skipped
• pnpm run typecheck
• pnpm run verify:package