Update lead custom fields

Each provenance entry has required basis (source_evidence, model_inference, user_assumption, unknown) and nonblank rationale (max 2,000 chars), plus optional sources (up to 20; default []). A source requires kind (url, crm_record, user_statement, tool_result, model_run) and nonblank reference (max 2,000 chars); optional excerpt is max 2,000 chars. source_evidence requires at least one source. Extra nested properties are rejected (malformed nested input receives 422). Provenance keys must be value keys, and null values cannot include provenance (semantic errors return 400). Empty values are accepted. The route validates custom field keys and values against this organization’s Lead definitions. The response is a normalized Lead record (including fields beyond this excerpt).