Providers omitting `finish_reason` in streaming chunks break the SDK with ZodError.

[LenAnderson]

One example is Google as a provider for Claude Models (e.g., Sonnet 4.5).

Thrown at @openrouter/sdk/0.8.0/esm/models/operations/sendchatcompletionrequest.js:37:72

ZodError: [
  {
    "code": "invalid_union",
    "errors": [
      [
        {
          "code": "invalid_value",
          "values": [
            "tool_calls"
          ],
          "path": [],
          "message": "Invalid input: expected \"tool_calls\""
        }
      ],
      [
        {
          "code": "invalid_value",
          "values": [
            "stop"
          ],
          "path": [],
          "message": "Invalid input: expected \"stop\""
        }
      ],
      [
        {
          "code": "invalid_value",
          "values": [
            "length"
          ],
          "path": [],
          "message": "Invalid input: expected \"length\""
        }
      ],
      [
        {
          "code": "invalid_value",
          "values": [
            "content_filter"
          ],
          "path": [],
          "message": "Invalid input: expected \"content_filter\""
        }
      ],
      [
        {
          "code": "invalid_value",
          "values": [
            "error"
          ],
          "path": [],
          "message": "Invalid input: expected \"error\""
        }
      ],
      [
        {
          "expected": "string",
          "code": "invalid_type",
          "path": [],
          "message": "Invalid input: expected string, received undefined"
        }
      ]
    ],
    "path": [
      "data",
      "choices",
      0,
      "finish_reason"
    ],
    "message": "Invalid input"
  }
]

Inspecting the rawEvent at that point shows that the property is indeed missing, and adding it (just setting finish_reason to null on the chunk) before the call to parse let's the SDK accept and process the chunks without complaint.