language_models: Filter out whitespace-only text content parts for OpenAI and OpenAI compatible providers

[Maokaman1]

Closes #40097

When multiple files are added sequentially to the agent panel, the request JSON incorrectly includes "text" elements containing only spaces. These empty elements cause the Zhipu AI API to return a "text cannot be empty" error.
The fix filters out any "text" elements that are empty or contain only whitespaces.

UI state when the error occurs:

Request JSON (causing the error):

{
  "model": "glm-4.6",
  "messages": [
    {
      "role": "system",
      "content": "<<CUT>>"
    },
    {
      "role": "user",
      "content": [
        {
          "type": "text",
          "text": "[@1.txt](zed:///agent/file?path=C%3A%5CTemp%5CTest%5C1.txt)"
        },
        { "type": "text", "text": " " },
        {
          "type": "text",
          "text": "[@2.txt](zed:///agent/file?path=C%3A%5CTemp%5CTest%5C2.txt)"
        },
        { "type": "text", "text": " describe" },

Release Notes:

• Filter out whitespace-only text content parts for OpenAI and OpenAI compatible providers