Fix KeyError 'text' on content_block_start in Anthropic streaming

Some Anthropic-compatible upstream providers (e.g. api-cc.freemodel.dev
proxying Claude Haiku 4.5) emit content_block_start events for text
blocks without the optional 'text' field, causing KeyError in the
chunk_parser. Per Anthropic's streaming spec the initial text on
content_block_start is conventionally empty (""), and the actual
content arrives via subsequent content_block_delta chunks regardless.

Use .get("text", "") so the parser tolerates the missing field
instead of aborting the stream with MidStreamFallbackError.

Reproduction:
  curl -sN -X POST http://proxy/v1/chat/completions \
    -H 'Authorization: Bearer $KEY' \
    -d '{"model":"claude-haiku-4-5","messages":[...],"stream":true}'

Before patch: stream aborts with KeyError: 'text' at handler.py:789
After patch: stream completes normally.

litellm/llms/anthropic/chat/handler.py

@@ -820,7 +820,12 @@ class ModelResponseIterator:
                     "type"
                 ]
                 if content_block_start["content_block"]["type"] == "text":
-                    text = content_block_start["content_block"]["text"]
+                    # Defensive .get() — some upstream Anthropic-compatible
+                    # providers omit the "text" field on content_block_start
+                    # for "text" blocks (spec says it should be ""). Fall back
+                    # to "" instead of raising KeyError, since the actual text
+                    # arrives via subsequent content_block_delta chunks anyway.
+                    text = content_block_start["content_block"].get("text", "")
                 elif (
                     content_block_start["content_block"]["type"] == "tool_use"
                     or content_block_start["content_block"]["type"] == "server_tool_use"

tests/test_litellm/llms/anthropic/chat/test_anthropic_chat_handler.py

@@ -1901,3 +1901,30 @@ def test_non_bash_tool_result_skipped():
     assert (
         len(code_results) == 0
     ), f"Expected 0 code_interpreter_results for text_editor result, got {len(code_results)}"
+
+
+def test_content_block_start_text_missing_text_field():
+    """
+    Regression test: some Anthropic-compatible upstreams omit the optional
+    "text" field on content_block_start events whose type == "text". Per
+    the Anthropic streaming spec the value is conventionally "", and the
+    real text arrives via subsequent content_block_delta chunks anyway,
+    so the parser must tolerate the missing field instead of raising
+    KeyError mid-stream.
+    """
+    chunk = {
+        "type": "content_block_start",
+        "index": 0,
+        # NOTE: "text" field intentionally omitted to mirror upstream payloads
+        # observed from Anthropic-compatible providers (e.g. proxies fronting
+        # claude-haiku-4-5-20251001) that skip the empty default.
+        "content_block": {"type": "text"},
+    }
+
+    iterator = ModelResponseIterator(None, sync_stream=True)
+
+    # Must not raise KeyError: 'text'
+    parsed = iterator.chunk_parser(chunk=chunk)
+
+    assert parsed is not None
+    assert parsed.choices[0].delta.content == ""