Adds integration test schema_tool_emits_error_v1_when_db_missing that
verifies NotIndexed errors are emitted as error.v1 JSON with isError=true.
Also fixes ErrorV1 struct to include required schema_version field per
error.v1 wire contract (docs/wire-schema/v1/error.schema.json).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>