Add SDK support for Cloud source execution

Track SDK support for the new Cloud source action endpoint, `POST /v1/sources/{sourceId}/execute`, once its platform specification is published.

The endpoint supports registry-declared JSON-only direct `get`, `list` and `api_search` operations across connectors/entities. Describe, writes, downloads, streaming, Context Store and unclassified operations are excluded.

Required `entity` and `action` are nonempty strings. `params` is an arbitrary JSON object, defaulting to `{}` when omitted. Optional `select_fields`/`exclude_fields` are nullable string arrays; `skip_truncation` is a strict boolean defaulting to `true`; `intent` is nullable with a 512-character maximum. Unknown root fields and invalid option types reject.

```json
{"entity":"repositories","action":"list","params":{"owner":"example","first":10},"skip_truncation":true}
```

The response requires `data` containing any JSON value, including objects, arrays, scalars or explicit null. Optional `meta` is a JSON object containing connector/pagination metadata. SDK serialization must preserve unknown/nested properties, numeric values, null-valued properties, empty objects/arrays, and required `data:null`; absent metadata stays absent. Explicit null is invalid for `params`, `skip_truncation` and `meta`.

Acceptance:

- Regenerate the Sources operation from the platform `api_sdk.yaml` using this repository's Speakeasy workflow.
- Verify request defaults/options and response wire contracts against the generated schema, including pagination, large numbers, nested nulls, empty `meta:{}`, and required `data:null`.
- Document Cloud authentication, workspace reader access, controlled availability, and the disabled/unavailable 503 response.
- Coordinate the SDK release with publication of the platform specification and endpoint activation.

The implementation ships disabled. The paired platform and Sonar changes passed 88/88 local E2E cases plus cleanup on their current unmerged revisions. See the [platform draft PR](https://github.com/airbytehq/airbyte-platform-internal/pull/19452) and its [paired premerge verification](https://github.com/airbytehq/airbyte-platform-internal/pull/19452#paired-premerge-proof). SDK regeneration and release remain pending.

Workspace discovery and describe/documentation/skill-discovery endpoints remain deferred. No SDK regeneration, release or production activation is claimed by this tracking update.