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)

src/grist_mcp/tools/filters.py

@@ -0,0 +1,37 @@
+"""Filter normalization for Grist API queries."""
+
+from typing import Any
+
+
+def normalize_filter_value(value: Any) -> list:
+    """Ensure a filter value is a list.
+
+    Grist API expects filter values to be arrays.
+
+    Args:
+        value: Single value or list of values.
+
+    Returns:
+        Value wrapped in list, or original list if already a list.
+    """
+    if isinstance(value, list):
+        return value
+    return [value]
+
+
+def normalize_filter(filter: dict | None) -> dict | None:
+    """Normalize filter values to array format for Grist API.
+
+    Grist expects all filter values to be arrays. This function
+    wraps single values in lists.
+
+    Args:
+        filter: Filter dict with column names as keys.
+
+    Returns:
+        Normalized filter dict, or None if input was None.
+    """
+    if not filter:
+        return filter
+
+    return {key: normalize_filter_value(value) for key, value in filter.items()}

src/grist_mcp/tools/read.py

@@ -2,6 +2,7 @@
 
 from grist_mcp.auth import Agent, Authenticator, Permission
 from grist_mcp.grist_client import GristClient
+from grist_mcp.tools.filters import normalize_filter
 
 
 async def list_tables(
@@ -56,7 +57,10 @@ async def get_records(
         doc = auth.get_document(document)
         client = GristClient(doc)
 
-    records = await client.get_records(table, filter=filter, sort=sort, limit=limit)
+    # Normalize filter values to array format for Grist API
+    normalized_filter = normalize_filter(filter)
+
+    records = await client.get_records(table, filter=normalized_filter, sort=sort, limit=limit)
     return {"records": records}