[backlog] In-route model config resets (upstream #409)

[DeliciousBuding]

    ## Source
    - Upstream: cita-777/metapi #409
    - Matrix: `docs/analysis/original-gap-matrix.md`
    - Category: bug-correctness
    - Status: unknown-needs-runtime · Priority: P0 · Backlog: yes
    - Epic: Admin UX
    - Parent: G3 #10

    ## Evidence (from matrix)
    Route update paths exist in admin token routes; in-route model config reset bug needs targeted repro.

    ## Acceptance criteria (draft — do not implement in G3)
    - [ ] Repro or refute model config reset on unrelated route edits

• Partial update semantics preserve model mapping / config fields not in payload

• API tests for partial vs full route update

• UI does not send empty model config on unrelated saves

    ## Out of scope for this issue filing
    Docs-only backlog. No product code until a later implementation wave claims this issue.