fix(openai-compat): restrict developer/assistant/system/tool messages to text-only content

**What:**
- Added OpenAIChatCompletionTextOnlyMessageContent type for text-only content validation
- Modified OpenAISystemMessageParam, OpenAIAssistantMessageParam, OpenAIDeveloperMessageParam, and OpenAIToolMessageParam to use text-only content type instead of mixed content
- OpenAIUserMessageParam unchanged - still accepts both text and images
- Updated OpenAPI spec files to reflect text-only content restrictions in schemas

**Why:**
- Enforces OpenAI API compatibility by restricting image content to user messages only
- Prevents API misuse where images might be sent in message types that don't support them
- Aligns with OpenAI's actual API behavior where only user messages can contain multimodal content
- Improves type safety and validation at the API boundary

**Test plan:**
- Added comprehensive parametrized tests covering all 5 OpenAI message types
- Tests verify text string acceptance for all message types
- Tests verify text list acceptance for all message types
- Tests verify image rejection for system/assistant/developer/tool messages (ValidationError expected)
- Tests verify user messages still accept images (backward compatibility maintained)

docs/_static/llama-stack-spec.yaml

@@ -6895,7 +6895,7 @@ components:
             - type: string
             - type: array
               items:
-                $ref: '#/components/schemas/OpenAIChatCompletionContentPartParam'
+                $ref: '#/components/schemas/OpenAIChatCompletionContentPartTextParam'
           description: The content of the model's response
         name:
           type: string
@@ -7037,7 +7037,7 @@ components:
             - type: string
             - type: array
               items:
-                $ref: '#/components/schemas/OpenAIChatCompletionContentPartParam'
+                $ref: '#/components/schemas/OpenAIChatCompletionContentPartTextParam'
           description: The content of the developer message
         name:
           type: string
@@ -7090,7 +7090,7 @@ components:
             - type: string
             - type: array
               items:
-                $ref: '#/components/schemas/OpenAIChatCompletionContentPartParam'
+                $ref: '#/components/schemas/OpenAIChatCompletionContentPartTextParam'
           description: >-
             The content of the "system prompt". If multiple system messages are provided,
             they are concatenated. The underlying Llama Stack code may also add other
@@ -7148,7 +7148,7 @@ components:
             - type: string
             - type: array
               items:
-                $ref: '#/components/schemas/OpenAIChatCompletionContentPartParam'
+                $ref: '#/components/schemas/OpenAIChatCompletionContentPartTextParam'
           description: The response content from the tool
       additionalProperties: false
       required: