source-hubspot: Reference for Flows v4 API errors — actionTypeId 500, type/objectTypeId 400, enum-operator, unified-events nesting

When the source-hubspot connector (or any downstream integration touching HubSpot's `POST /automation/v4/flows` endpoint) errors, the HubSpot-side messages are misleading enough that they cost real debugging time. Sharing a reproducible reference in case it's useful for connector error-mapping, retry logic, or docs.

Tested on a Marketing Hub Pro portal, 30/07/2026, 74 live flows enumerated. Four documented gotchas:

1. **Legacy `actionTypeId: "0-13"` (alter-list-membership) returns HTTP 500** with an empty body. HubSpot split it — use `"0-63809083"` (add to list) or `"0-63863438"` (remove), both with `fields.listId` + `fields.targetObject: "{{ enrolled_object }}"`.
2. **Top-level `type` is required and must match `objectTypeId`.** Omitting it 400s with `Some required fields were not set: [type]`; mismatch 400s with `Expected an object type other than "0-1" objectType but got "0-1"` — blames `objectTypeId` but the culprit is `type`. `CONTACT_FLOW` for `0-1`, `PLATFORM_FLOW` for `0-3`.
3. **ENUMERATION-property operator allowlist is narrower than CRM Search.** `IS_NOT_EQUAL_TO` 400s — use `IS_NONE_OF` with a single-element `values` array; add `includeObjectsWithNoValueSet: true` if nulls should match.
4. **Unified-events filter branches cannot nest.** Nested `filterBranches[]` inside a `UNIFIED_EVENTS` entry returns 400 — promote each condition to its own top-level `eventFilterBranches[]` entry (implicitly OR'd).

Full reproducible request/response pairs, allowlist table and verification recipe: https://www.elemra.com/learn/hubspot-flows-v4-api-errors

Also cross-referenced upstream in the official SDK repos: [HubSpot/hubspot-api-nodejs#695](https://github.com/HubSpot/hubspot-api-nodejs/issues/695) and [HubSpot/hubspot-api-python#498](https://github.com/HubSpot/hubspot-api-python/issues/498).

Not proposing a specific connector change — more a heads-up in case the errors show up in support tickets or you want to fold clearer messaging into the source-hubspot streams that touch these endpoints.