Update Data Connectors schema: remove source, add state_version, fix default_value

[ellicenelson]

Why?

Three doc fixes for the Data Connectors public API to match monolith behavior:

1. default_value no longer needs nullable: true on the create schema — it defaults to "" when omitted
2. source was an internal field leaked in the detail response — now removed from the monolith presenter
3. state_version query parameter on GET show was undocumented, along with its 400 error response

How?

Update the OpenAPI spec: remove nullable from default_value in create schema, remove source from detail schema and examples, add state_version query param and 400 error to GET /data_connectors/{id}.

Related PRs

• Monolith: https://github.com/intercom/intercom/pull/487069
• Developer docs: https://github.com/intercom/developer-docs/pull/789

_{Generated with Claude Code}