Add an optional ``precheck`` method to ``OperationValidatorExtension`` that
extensions can override to gate a request *before* its body is read off the
wire. Wire it as a FastAPI ``Depends`` ahead of the body parameter on the
billable POST routes (retain, recall, reflect, file retain, mental-model
create, mental-model refresh) so a rejecting precheck short-circuits the
request without ever materialising the JSON payload in memory.

The post-body-parse ``validate_retain`` / ``validate_recall`` /
``validate_reflect`` hooks are unchanged and remain the source of truth for
precise per-call cost and quota arithmetic. ``precheck`` is intentionally a
cheap, side-effect-free check — its sole purpose is to let an extension
short-circuit work that would otherwise allocate the request body
unnecessarily (e.g. a quota-exhausted caller submitting many large bodies).

Why before body parse:

FastAPI resolves dependencies before deserialising the route's body
parameter. A validator that runs only after parse — i.e. inside the route
handler's body — sees the already-materialised request, which is the wrong
layer for "this caller should not be allowed to spend resources on this
request at all" decisions. Wiring as ``Depends`` puts the gate at the right
layer with a one-line change per route.

Verified:

- FastAPI 0.125.0 resolves ``Depends`` raising ``HTTPException`` before
  Pydantic deserialises the body, regardless of declaration order. A
  reproducer using a ``model_validator(mode='before')`` recorder confirms
  zero body-parse calls on the rejection path.
- The new ``PrecheckContext`` carries only operation name + bank_id +
  request_context (already-resolved tenant). No body access — by design.
- Default ``precheck`` returns ``ValidationResult.accept()``; existing
  validators are unaffected.

Tests: +7 unit tests covering the default no-op, the FastAPI Depends
wiring, accept/reject paths, status-code/reason propagation, and explicit
"body never parsed on rejection" assertions for retain / recall / reflect
plus a "GET routes are unaffected" guard. All passing.