test(ai): add contract tests for resolveUsedFraction

[oldschoola]

Problem

resolveUsedFraction is the shared helper that resolves a used fraction (0..1) from whichever UsageAmount fields a provider populated. It's used by the CLI usage renderer, the TUI usage renderer (via PR #3317), and the credential ranking strategy — but had zero dedicated test coverage.

Changes

Added 12 contract tests covering:

• All four resolution paths (explicit usedFraction, used/limit, percent+used, inverted remainingFraction)
• Precedence ordering (usedFraction > used/limit > percent+used > remainingFraction)
• Edge cases: limit=0, overage (usedFraction > 1), remainingFraction=0, no resolvable fields
• The remainingFraction fallback that was missing from the TUI's local copy (fixed in fix(usage): use shared resolveUsedFraction in TUI path, fixing missing remainingFraction fallback #3317)

Tests

• 12/12 pass

[roboomp]

lgtm — incremental, well-scoped test addition.

• Pins all four resolveUsedFraction resolution paths (usage.ts:94-103), the precedence order, and the edge cases (limit=0, overage, remainingFraction=0, clamp at 0, unresolvable input).
• Local makeLimit helper, contract-level assertions on the return value, no mock.module() / globals — full-suite-safe.
• CHANGELOG entry under [Unreleased]; 12/12 pass locally.

resolveUsedFraction is consumed by usage-cli.ts (status, summary, render bar, history bucketing) and auth-storage.ts:2232; pinning the precedence is a useful regression net after the TUI's local copy missed the remainingFraction fallback in #3317. Thanks @oldschoola.