feat(aws-lambda): validate variables + 256 KiB Step Functions input cap (#976)

Add client-side validation for the new config.variables field
(introduced in PR 9.1) and a 256 KiB cap on the full Step Functions
Standard execution input. Both checks throw a typed InvalidConfigError
BEFORE the SDK calls StartExecution — catching the obvious mistakes
locally instead of as a States.DataLimitExceeded 50 ms into the
execution.

validateVariablesPayload walks the variables tree and rejects:
- functions, Symbols, BigInts, non-finite numbers
- undefined leaves (silently dropped by JSON.stringify — would
  surprise the caller when their value doesn't show up in the render)
- non-plain objects (Date, Map, class instances) — Date's toJSON does
  round-trip as a string, but the composition gets a string, not a
  Date, so explicit reject is clearer

validateStepFunctionsInputSize measures the actual UTF-8 byte length
of JSON.stringify(input) against the 256 KiB cap. We use Standard
workflows (per the plan §6.2 / §15.2) for execution-history
visibility, so the cap is 256 KiB (Express would be 32 KiB). The error
message names the actual byte count, the cap, and points at the
templates-on-lambda#working-with-large-variables section so users
know to URL-reference media assets instead of inlining them.

Both helpers are exported from @hyperframes/aws-lambda/sdk so adapters
that build custom Step Functions inputs (batch verbs, future Temporal
ports) can reuse the same gates.

Phase 9 PR 9.2 of the distributed rendering plan.

packages/aws-lambda/src/sdk/index.ts

@@ -22,6 +22,12 @@ export {
   computeRenderCost,
   type RenderCost,
 } from "./costAccounting.js";
-export { InvalidConfigError, validateDistributedRenderConfig } from "./validateConfig.js";
+export {
+  InvalidConfigError,
+  MAX_STEP_FUNCTIONS_INPUT_BYTES,
+  validateDistributedRenderConfig,
+  validateStepFunctionsInputSize,
+  validateVariablesPayload,
+} from "./validateConfig.js";
 export type { SerializableDistributedRenderConfig } from "../events.js";
 export type { DistributedFormat } from "../formatExtension.js";