fix(openai-compat): restrict developer/assistant/system/tool messages to text-only content (#2932)

**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

closes #2894 

**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)

llama_stack/apis/inference/inference.py

@@ -464,6 +464,8 @@ register_schema(OpenAIChatCompletionContentPartParam, name="OpenAIChatCompletion
 
 OpenAIChatCompletionMessageContent = str | list[OpenAIChatCompletionContentPartParam]
 
+OpenAIChatCompletionTextOnlyMessageContent = str | list[OpenAIChatCompletionContentPartTextParam]
+
 
 @json_schema_type
 class OpenAIUserMessageParam(BaseModel):
@@ -489,7 +491,7 @@ class OpenAISystemMessageParam(BaseModel):
     """
 
     role: Literal["system"] = "system"
-    content: OpenAIChatCompletionMessageContent
+    content: OpenAIChatCompletionTextOnlyMessageContent
     name: str | None = None
 
 
@@ -518,7 +520,7 @@ class OpenAIAssistantMessageParam(BaseModel):
     """
 
     role: Literal["assistant"] = "assistant"
-    content: OpenAIChatCompletionMessageContent | None = None
+    content: OpenAIChatCompletionTextOnlyMessageContent | None = None
     name: str | None = None
     tool_calls: list[OpenAIChatCompletionToolCall] | None = None
 
@@ -534,7 +536,7 @@ class OpenAIToolMessageParam(BaseModel):
 
     role: Literal["tool"] = "tool"
     tool_call_id: str
-    content: OpenAIChatCompletionMessageContent
+    content: OpenAIChatCompletionTextOnlyMessageContent
 
 
 @json_schema_type
@@ -547,7 +549,7 @@ class OpenAIDeveloperMessageParam(BaseModel):
     """
 
     role: Literal["developer"] = "developer"
-    content: OpenAIChatCompletionMessageContent
+    content: OpenAIChatCompletionTextOnlyMessageContent
     name: str | None = None