@ai-sdk/google stopped working with function calling in production

### Description

I've been using google function calling via their models, it was working fine yesterday, nothing in the code changed and suddenly it stopped working due to zod schema.

Expected functionality: streaming tool calling as usual.

Actual behavior:
Error (it is replicated on the pro model):
`Streaming error: [Error [AI_TypeValidationError]: Type validation failed: Value: {"candidates":[{"content":{"parts":[{"thought":true,"thoughtSignature":"AVSoXO7qF/Am002atYzdg3gH+fuKwqaFBtNL8wTanfsv1JycHM8HNKOGp8zmJJBoE6twT5wRnwaj9/yrr2HcQK57qc/cWKQ="}],"role":"model"},"index":0}],"usageMetadata":{"promptTokenCount":7327,"totalTokenCount":7407,"promptTokensDetails":[{"modality":"TEXT","tokenCount":7327}],"thoughtsTokenCount":80},"modelVersion":"models/gemini-2.5-flash-preview-04-17","responseId":"4PI9aOGZIpK1xN8P4NHFwQk"}.
Error message: [
  {
    "code": "invalid_union",
    "unionErrors": [
      {
        "issues": [
          {
            "code": "invalid_type",
            "expected": "string",
            "received": "undefined",
            "path": [
              "candidates",
              0,
              "content",
              "parts",
              0,
              "text"
            ],
            "message": "Required"
          }
        ],
        "name": "ZodError"
      },
      {
        "issues": [
          {
            "code": "invalid_type",
            "expected": "object",
            "received": "undefined",
            "path": [
              "candidates",
              0,
              "content",
              "parts",
              0,
              "functionCall"
            ],
            "message": "Required"
          }
        ],
        "name": "ZodError"
      },
      {
        "issues": [
          {
            "code": "invalid_type",
            "expected": "object",
            "received": "undefined",
            "path": [
              "candidates",
              0,
              "content",
              "parts",
              0,
              "inlineData"
            ],
            "message": "Required"
          }
        ],
        "name": "ZodError"
      }
    ],
    "path": [
      "candidates",
      0,
      "content",
      "parts",
      0
    ],
    "message": "Invalid input"
  }
]] {
  value: [Object],
  [cause]: [Error [ZodError]: [
    {
      "code": "invalid_union",
      "unionErrors": [
        {
          "issues": [
            {
              "code": "invalid_type",
              "expected": "string",
              "received": "undefined",
              "path": [
                "candidates",
                0,
                "content",
                "parts",
                0,
                "text"
              ],
              "message": "Required"
            }
          ],
          "name": "ZodError"
        },
        {
          "issues": [
            {
              "code": "invalid_type",
              "expected": "object",
              "received": "undefined",
              "path": [
                "candidates",
                0,
                "content",
                "parts",
                0,
                "functionCall"
              ],
              "message": "Required"
            }
          ],
          "name": "ZodError"
        },
        {
          "issues": [
            {
              "code": "invalid_type",
              "expected": "object",
              "received": "undefined",
              "path": [
                "candidates",
                0,
                "content",
                "parts",
                0,
                "inlineData"
              ],
              "message": "Required"
            }
          ],
          "name": "ZodError"
        }
      ],
      "path": [
        "candidates",
        0,
        "content",
        "parts",
        0
      ],
      "message": "Invalid input"
    }
  ]] {
    issues: [ [Object] ],
    addIssue: [Function (anonymous)],
    addIssues: [Function (anonymous)]
  }
}`


Changing the provider to openai solves it immediately, removing the function calling also solves it but breaks the intended functionality. reverted to openai for now.

Please help, I have not changed anything in the code, but it got broken.

### AI SDK Version

"@ai-sdk/google": "^1.2.16",
"@ai-sdk/openai": "^1.3.22",
"@ai-sdk/provider": "^1.1.3",
"@ai-sdk/provider-utils": "^2.2.8",
"@ai-sdk/react": "^1.2.11",

Labels: bug, ai/provider, provider/google, provider/openai
State: closed