fix(usage): record exact embedding tokens

[zierocode]

Problem

Successful embedding responses already return provider usage to clients, but the local usage history does not record it. Embedding traffic is therefore absent from usage and cost accounting.

Fix

• return normalized provider usage alongside the core success response
• persist usage after a successful embedding request
• accept only exact positive safe-integer totals where input equals total and completion is zero
• reject estimated, malformed, zero, or internally inconsistent usage instead of fabricating counts
• keep persistence non-blocking so a DB failure cannot fail a successful inference

Scope

This is provider-neutral embedding accounting. It does not change response bodies, provider routing, credentials, or the xAI/cache-cost work in #2453.

TDD evidence

RED on 79918c7830695bbca4a45c9fea4a42c3e9fd73d1:

records exact provider usage for successful embedding requests
Number of calls: 0

GREEN on 660552583fca0df6c51cbd23312b016ac5e05c32:

• focused tests: 2 files, 47 tests passed
• ESLint: passed on all changed files
• production build: passed with isolated DATA_DIR

[decolua]

Thanks @zierocode for the contribution! Reviewed and merged into master. 🙏