…coredev#2029)

## Summary
Fixes missing documentation in OpenAPI schema generation for both
field-level and struct-level comments.

## Problems Fixed

### 1. Field documentation lost with schema references
When a struct field with documentation references another struct type,
the field documentation was lost because `val.Value` is `nil` for schema
references.

**Before:**
```yaml
customer:
  $ref: "#/components/schemas/User"
  # Field documentation was lost
```

**After:**
```yaml
customer:
  description: "Customer who placed the order (not the same as delivery recipient)"
  allOf:
    - $ref: "#/components/schemas/User"
```

### 2. Missing struct-level documentation
Struct declarations with documentation comments weren't including that
documentation in the generated OpenAPI component schemas.

**Before:**
```yaml
"svc.User": {
  "type": "object",
  "properties": {...}
  # Missing title/description from struct docs
}
```

**After:**
```yaml
"svc.DocumentedUser": {
  "type": "object", 
  "title": "DocumentedUser represents a user in the system with profile information",
  "properties": {...}
}
```

## Technical Details

### Field Documentation Fix (lines 75-87)
Uses the `allOf` pattern recommended for OpenAPI 3.0 to add descriptions
to `$ref`. This is the [official
workaround](OAI/OpenAPI-Specification#2033)
for OpenAPI 3.0's limitation where sibling properties to `$ref` are
ignored.

### Struct Documentation Fix (lines 288-302) 
When generating named schema types, include the declaration's
documentation from `g.md.Decls[typ.Id].Doc` in the schema's title and
description fields.

## Testing
- Added documented struct types (`DocumentedUser`, `DocumentedOrder`) to
test data
- Updated all expected test outputs (OpenAPI, TypeScript, JavaScript,
Go)
- Verified both fixes work together in generated specs
- All existing tests pass

## Example Output
The generated OpenAPI now includes comprehensive documentation:

```json
{
  "components": {
    "schemas": {
      "svc.DocumentedUser": {
        "title": "DocumentedUser represents a user in the system with profile information", 
        "properties": {
          "name": {"type": "string"},
          "email": {"type": "string"}
        }
      }
    }
  },
  "paths": {
    "/svc.CreateDocumentedOrder": {
      "requestBody": {
        "schema": {
          "properties": {
            "customer": {
              "allOf": [{"$ref": "#/components/schemas/svc.DocumentedUser"}],
              "description": "Customer who placed this order (different from shipping recipient)"
            }
          }
        }
      }
    }
  }
}
```

This resolves the issue where OpenAPI client generators and
documentation tools couldn't access important contextual information
about API fields and schemas.

---

I ran this against our own encore project and it works as expected.