feat: add top-level description field to OAS (#268)

#### Description of changes

Added a description field to `RuntimeConfig` with a default of `""`, and
set a kwarg in the `FastAPI()` constructor called in
`runtime.serve.create_restapi()` of `description=config.description`.

This inserts the Tesseract description from `tesseract_config.yaml`,
already present as an environment variable `TESSERACT_DESCRIPTION` in
built Tesseract containers, into the top-level of the OpenAPI
Specification.

#### To do

- [x] Investigate why newline characters appear to have their
backslashes escaped in the description field

#### Testing done

Manually tested.

<img width="1920" height="511" alt="image"
src="https://github.com/user-attachments/assets/984e2d7f-d030-4ce8-8725-91995b4b8fad"
/>

<img width="1898" height="519" alt="image"
src="https://github.com/user-attachments/assets/8b55356e-c591-4504-9a0d-42d697a55ae1"
/>

Author: jacanchaplais

tesseract_core/runtime/config.py

@@ -13,6 +13,7 @@ class RuntimeConfig(BaseModel):
 
     api_path: FilePath = Path("tesseract_api.py")
     name: str = "Tesseract"
+    description: str = ""
     version: str = "0+unknown"
     debug: bool = False
     input_path: str = "."

tesseract_core/runtime/serve.py

@@ -38,6 +38,7 @@ def create_rest_api(api_module: ModuleType) -> FastAPI:
     app = FastAPI(
         title=config.name,
         version=config.version,
+        description=config.description.replace("\\n", "\n"),
         docs_url=None,
         redoc_url="/docs",
         debug=config.debug,