fix(claude): synthesize belated tool_use starts for empty-name calls

- Emit delayed `content_block_start` for tool calls that arrive with missing names but still carry `id`/arguments, using fallback `tool_<index>` names.
- Keep dropping only fully empty signals (no `id`, no `name`, no `arguments`) so legitimate streams are preserved and `tool_use` stop-reason behavior remains correct.

Closes: #3748