fix anthropic streaming content_block_start struct bug

[huangwei1024]

opencode Error message: [
{
"expected": "string",
"code": "invalid_type",
"path": [
"content_block",
"text"
],
"message": "Invalid input: expected string, received undefined"
}
]

---

-- it is claude exmple

event: content_block_start
data: {"type": "content_block_start", "index": 0, "content_block": {"type": "text", "text": ""}}

[cla-assistant]

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