Add integration tests

[GavinRay97]

Comprehensive plan/spec committed to integration-tests/INTEGRATION-TESTS-PLAN-PROMPT.md for posterity.

Uses local ddn CLI to create a temp local supergraph and add the nodejs-lambda connector to it.
See the lifecycle description below:

  **beforeAll** (runs once):
  1. Build the ndc-lambda-sdk (`npm run build` in `../ndc-lambda-sdk`)
  2. Create temp directory for DDN project
  3. `ddn supergraph init <tempdir>/test-project`
  4. `ddn connector init myjs --hub-connector hasura/nodejs --subgraph .../app/subgraph.yaml --configure-port 9876`
  5. Copy `fixtures/functions.ts` into connector directory
  6. Patch connector's `package.json` to use `file:` reference to local SDK
  7. `npm install` in connector directory
  8. Start connector via `node ndc-lambda-sdk/bin/index.js host -f functions.ts serve --configuration ./ --port 9876`
  9. Poll `/health` until 200 OK (30s timeout)

Example output:

ndc-nodejs-lambda/integration-tests on  gavin/integration-tests via  v22.14.0
❯ npm test

> ndc-nodejs-lambda-integration-tests@1.0.0 test
> mocha

    Building ndc-lambda-sdk...
    Initializing DDN project in /tmp/ndc-lambda-integration-h7y2q6/test-project...
    Initializing connector...
    Installing connector dependencies...
    Starting connector server...
    Waiting for connector to be healthy...
    Connector is healthy.
    Updating DataConnectorLink schema...
    DataConnectorLink schema updated.
  Capabilities endpoint
    ✔ GET /capabilities returns query and mutation capabilities

  DDN supergraph build
    ✔ successfully builds the supergraph locally (1271ms)
    ✔ produces build artifacts

  DDN connector introspect
    ✔ DataConnectorLink has valid schema after manual introspection

  Error handling
    ✔ throwForbidden returns 403
    ✔ throwConflict returns 409
    ✔ throwUnprocessable returns 422
    ✔ throwInternalError returns 500
    ✔ querying a nonexistent function returns 400
    ✔ calling a procedure via query endpoint returns 400

  Health endpoint
    ✔ GET /health returns 200

  Mutation tests
    counter procedures
      ✔ resetCounter resets to zero
      ✔ incrementCounter increments the counter
      ✔ counter state persists across calls
    createUser procedure
      ✔ creates a user with name and email
      ✔ returns user with id field
    async procedure
      ✔ asyncCreateItem creates an item asynchronously

  Query tests
    scalar types
      ✔ hello with no args returns default greeting
      ✔ hello with name arg returns personalized greeting
      ✔ add returns sum of two numbers
      ✔ isTrue returns boolean correctly
      ✔ echoBigInt echoes back a bigint value
    nullable and optional arguments
      ✔ greetNullable with null returns anonymous
      ✔ greetNullable with a value returns greeting
      ✔ greetOptional with no args returns default
      ✔ greetOptional with a value returns greeting
    array types
      ✔ sumArray returns sum of number array
      ✔ reverseStrings returns reversed array
    object types
      ✔ getDistance computes distance between coordinates
      ✔ describePlace returns a description string
    nested return types
      ✔ getPersonWithAddress returns nested object with field selection
    async functions
      ✔ asyncGreet returns async greeting
      ✔ asyncGetPlace returns async place object
    variables
      ✔ executes query with variable arguments for each variable set
      ✔ mixes literal and variable arguments

  Schema endpoint
    ✔ returns functions and procedures
    ✔ contains scalar type definitions
    ✔ contains object type definitions
    functions (queries)
      ✔ includes hello function
      ✔ includes add function
      ✔ includes isTrue function
      ✔ includes echoBigInt function
      ✔ includes nullable and optional argument functions
      ✔ includes object-arg functions
      ✔ includes array functions
      ✔ includes async functions
      ✔ includes error-throwing functions
    procedures (mutations)
      ✔ includes incrementCounter
      ✔ includes resetCounter
      ✔ includes createUser
      ✔ includes asyncCreateItem

    Stopping connector server...
    Cleaning up temp directory...

  51 passing (16s)