feat: normalize filter values to array format for Grist API

The Grist API requires all filter values to be arrays. This change adds
automatic normalization of filter values in get_records, wrapping single
values in lists before sending to the API.

This fixes 400 errors when filtering on Ref columns with single integer IDs.

Changes:
- Add filters.py module with normalize_filter function
- Update get_records to normalize filters before API call
- Add Orders table with Ref column to mock Grist server
- Add filter validation to mock server (rejects non-array values)
- Fix shell script shebangs for portability (#!/usr/bin/env bash)

scripts/get-test-instance-id.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # scripts/get-test-instance-id.sh
 # Generate a unique instance ID from git branch for parallel test isolation
 

scripts/run-integration-tests.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # scripts/run-integration-tests.sh
 # Run integration tests with branch isolation and dynamic port discovery
 set -e