Tool approval HMAC payload now uses JSON.stringify to prevent collision

Patch Changes

• 7fa85b2: fix(ai): use injective serialization for tool approval HMAC payload

  The tool approval signature (experimental_toolApprovalSecret) built its HMAC payload by joining fields with \n. Because fields such as toolName and toolCallId can themselves contain a newline, distinct field tuples could serialize to identical bytes, allowing a signed approval to verify against a different tuple. The payload is now serialized with JSON.stringify (with a versioned domain-separation prefix), which escapes delimiter/control characters and makes the encoding injective.

  Verification remains backwards compatible: a signature in the old format still verifies, but only when no field contains the \n delimiter (the condition that made the old format ambiguous), so a pending approval that straddles an upgrade is not rejected while the collision stays closed.