BOLT-1068: Preserve FranklinWH isTest flag in site grant credentials

Artem asked in Slack thread 1780036222.881059 / message 1780063382.528519 to create a PR for the FranklinWH grid-services test credential bug. Scope: domains/site only. Add isTest: z.boolean().optional() to FranklinWHCredentialsSchema so mapGrantDaoToEntity/parseCredentials preserves DB credentials.isTest=true; add a unit test proving isTest is retained; run domains/site lint/build or targeted tests as feasible; open PR linked to BOLT-1068; monitor CI. Branch: artem-agent/bolt-1068-franklinwh-istest.