feat(api): generate and migrate Prompt REST resources (#750)

### AI Summary

Implements the Prompt slice of #683 by publishing all six operations
tagged Prompts and moving `load_prompt()` retrieval onto the public REST
resources. Prompt request and response models are also exported through
the public API.

### Migration flow

```text
Before
  load_prompt(id=...) ------> raw GET /v1/prompt/{id}
  load_prompt(slug=...) ----> raw GET /v1/prompt

After
  load_prompt(id=...) ------> generated GET /v1/prompt/{id}
  load_prompt(slug=...) ----> generated GET /v1/prompt
```

| SDK workflow | Previous wire call | Public REST call |
| --- | --- | --- |
| Load by ID | Raw `GET /v1/prompt/{id}` | Generated `GET
/v1/prompt/{id}` | | Load by project and slug | Raw `GET /v1/prompt` |
Generated `GET /v1/prompt` | | Prompt CRUD | No generated resource |
`client.openapi.prompts` |

| Behavior | Result |
| --- | --- |
| Explicit credentials | Use isolated, cached generated clients without
mutating global login | | Active global login | Reuse the existing
generated API client | | Version and environment | Explicit version
continues to take precedence over environment | | Cache fallback |
Preserve local prompt cache fallback for retryable server failures | |
Lazy and async loading | Continue deferring retrieval until the prompt
is used | | Prompt lookup by ID | Consume the direct response without a
synthetic `objects` envelope | | Retry behavior | GET operations are
safe reads; version-producing writes are not retried |

Refs #683

Author: AbhiPrasad

openapi/README.md

@@ -16,7 +16,7 @@ make check-api-client-codegen
 ```
 
 The check regenerates in a temporary directory and reports drift without changing the worktree.
-Currently selected tags are Projects, Experiments, and Datasets. Each tag produces one resource and
+Currently selected tags are Projects, Experiments, Datasets, and Prompts. Each tag produces one resource and
 operation registry. Models used by one resource stay in that resource's model module; shared models
 live in `models/common.py`; unreachable models are omitted.
 

openapi/config.json

@@ -32,7 +32,8 @@
     "generated_tags": [
       "Projects",
       "Experiments",
-      "Datasets"
+      "Datasets",
+      "Prompts"
     ],
     "safe_reads": [
       "postExperimentIdFetch",