fix(openapi): declare Base Fee response contract

[epistemedeus]

What

Give GET /v1/q/base-fee a truthful machine-readable success contract in both
the Bazaar discovery metadata and generated OpenAPI document.

The runtime already guarantees these values on every successful fresh, cached,
and blob-less response:

• as_of_ms: integer
• block: integer
• source: string

The OpenAPI generator now projects a response schema only when seller discovery
declares a non-empty required array, and copies only those required property
definitions. That keeps optional RPC-dependent fields out of the guaranteed
response contract.

Scope

• No route, handler, price, payment, settlement, facilitator, or MIME behavior
  changes.
• Every unrelated paid operation keeps its existing untyped
  application/json success response.
• CSV and JSONL response modeling is intentionally out of scope.

Verification

• Full suite: 631 passed, 0 failed across 30 suites.
• Focused OpenAPI, fee, and x402 suites pass.
• Generated document audit inspected all 33 paid operations. Only Base Fee is
  strengthened, with properties and required both exactly
  as_of_ms, block, and source.
• git diff --check passes.

The upstream main branch's current npm ci failure from the existing ws
override and lockfile mismatch predates this change. The full suite passes with
the repository dependencies installed without changing the lockfile.