A continuation SSE delta that repeats a tool call's `id` or `name` as an
empty string — or as `null`, which some OpenAI-compatible gateways send —
erased the identity established by the call's first delta. The assembled
block reached the loop with an empty name and failed as `unknown tool ""`,
and the empty `callId` persisted into `tool/result`, which the session
reader refuses on reopen.
`acceptIdentity` accepts only a non-empty string, so a repeated empty or
null field means "no update". A tool call still missing `id` or `name` at
`[DONE]` ends the response with the new retryable `MALFORMED_TOOL_CALL`
code instead of closing an unusable block.
