fix: preserve usage account attribution across retries

[yangeni]

Summary

• Keep request token usage attached to the account that produced the usage callback.
• Do not let a later retry or selector snapshot overwrite that account with another account.
• Preserve selector fallback only when no usage callback exists or the callback AuthID matches.
• Keep mismatched or unmapped usage unattributed rather than charging a different account.

Scope

• Does not change routing, retries, model pricing, quota querying, WebSocket execution IDs, or existing request logs.
• Does not rewrite historical billing records; historical reassignment requires original request-level evidence.
• No credentials or private account data are included.

Verification

• go test ./...
• go test -race ./... for the affected package
• Windows amd64 cross-compile test completed locally; CI remains the runtime verification surface.

The regression tests cover selector changes after usage, missing selector state, matching AuthID fallback, multiple successful usage records, and API-key attribution.