In runMiddleware, when a middleware function returned an object (early
result), it was returned unwrapped. lambdaHandler then assigned
`resultMiddlewareFns = middlewareResult.middleware` which resolved to
`undefined`, causing `TypeError: resultMiddlewareFns is not iterable`
in the catch block on subsequent errors.

Fixed by wrapping the early return as `{ status: 'result', result }`.

Also adds:
- runMiddleware unit tests (empty list, callbacks, early return,
  middleware throw, callback flush on throw)
- streamHandler.test.ts covering isStreamRequestHandler,
  createStreamRequestHandler (fromEvent, buildHTTPMetadata,
  toErrorResult), and stream lambda handler integration paths

Add awslambda and streamify to cspell-dict.txt, and apply prettier
formatting to middleware.test.ts, streamHandler.test.ts, and _create.ts.

StreamRequestHandler.toErrorResult is async and returns {metadata, body},
not HttpMetadata. Calling it without await in runMiddleware's catch block
produced a Promise where HttpMetadata was expected, causing undefined
behaviour when middleware throws in stream mode.

Fix by re-throwing for stream handlers — the stream path in lambdaHandler
already has its own catch that properly awaits toErrorResult and writes
the error response to the stream. Adds a test to cover this path.

Adds a mock HTTP adapter and integration test that run Apollo's full
defineIntegrationTestSuite against the V2 stream handler (with and
without base64 encoding). Also fixes buildHTTPMetadata to include
content-length for complete responses, matching the non-streaming handlers.