Dry-run a caller identity before creating it

Validates the exact body POST /v1/branded-calling/identities takes and runs the same deterministic lints the review runs on it without creating anything, with the same codes and fields the queued identity's findings carry. A block finding is what the review would bounce (two references sharing a phone, a reference inside the business, an invalid timezone); a warn finding slows vetting (a display name that does not read as the business, a call reason outside the carrier catalogue, a public-mailbox authorizer, a logo that does not answer). ok is true when there is no block.