Streaming: content_block_start reuses indices after web_search_tool_results

[mkdynamic]

This appears to be an API bug (not specific to the TS SDK).

Minimal reproduction case:

curl -X POST https://api.anthropic.com/v1/messages \
  -H "x-api-key: $ANTHROPIC_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "anthropic-beta: web-search-2025-03-05" \
  -H "content-type: application/json" \
  -d '{"model":"claude-haiku-4-5","max_tokens":1024,"stream":true,"tools":[{"type":"web_search_20250305","name":"web_search"}],"messages":[{"role":"user","content":"Population of Spain?"}]}'

Observed:

content_block_start index=0 server_tool_use
content_block_start index=1 web_search_tool_result
content_block_start index=1 text  ← duplicate

Expected: Sequential unique indices (0, 1, 2, ...)

Per the docs:

Each content block will have an index that corresponds to its index in the final Message content array.

https://platform.claude.com/docs/en/api/messages-streaming

The streaming documentation shows web_search responses with sequential indices (0, 1, 2, 3...). The observed duplicate indices violate this contract.