fix(home): preserve requested usage metadata before auth preparation

Home execution built its executor context without the request metadata
enrichment used by the normal execution path, so usage records fell back
to defaults: an explicit `service_tier: priority` request was serialized
as `default`, the requested model alias degraded to the execution model,
and an explicit `generate: false` became `true`.

Enrich the Home attempt context in `executeHome` before auth preparation,
execution, and Home result reporting. Apply the same enrichment in
`executeStreamMixedOnce`, where prepare-stage usage records had an
identical gap because enrichment only happened downstream in
`executeStreamWithModelPool`, after `prepareHomeRequestAuth` had already
reported the `prepare_failed` result.

Fixes #4791