Parses the JMAP Session object (RFC 8620 §2): fetches GET {jmapUrl},
extracts apiUrl and primary accountId, and wraps API calls via
call(methodCalls) which POSTs to apiUrl with Basic Auth.
Handles relative apiUrl, primaryAccounts fallback, and top-level
JMAP error responses. Covered by unit tests.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>