/api/keywords parameter validation does not enforce documented min/max for size

## Description
The `size` parameter on `/api/keywords` does not enforce the documented valid range of 1-1000.

## Steps to Reproduce
GET https://catalog.data.gov/api/keywords?size=0 (returns 1 result, not 0 or an error)
GET https://catalog.data.gov/api/keywords?size=1001 (returns 1000 results silently)

## Expected Behavior
Per the OpenAPI spec, valid range is 1-1000. Values outside this range should return HTTP 422.

## Actual Behavior
- `size=0` returns 1 result. Response `size` field shows `1`, not `0`.
- `size=1001` returns 1000 results with no indication the value was out of range.

## Notes
Confirmed April 27, 2026. If silent clamping is intentional, the behavior should be documented.