Add a repeatable attr query parameter to the GET /executions endpoint that
parses key-only (exists check) and key:value (exact or wildcard-via-*)
filters. Invalid keys are mapped to HTTP 400 via ResponseStatusException.
The POST /executions/search path already honoured attributeFilters from
the request body via the Jackson canonical ctor; an IT now proves it.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>