fix(sse): emit empty id field to reset Last-Event-ID

[lasder-ca]

Summary

Emit an SSE id field when SSEMessage.id is an empty string.

Problem

SSEStreamingApi.writeSSE() currently uses a truthy check when serializing message.id:

message.id && \id: ${message.id}``

This omits the field when id is an empty string.

In Server-Sent Events, an empty id field resets the client's last event ID. After the reset, a subsequent reconnect should not send a Last-Event-ID header.

Hono currently cannot express this behavior through writeSSE().

Specification:
https://html.spec.whatwg.org/multipage/server-sent-events.html#the-last-event-id-header

Change

Use an explicit undefined check:

message.id !== undefined && \id: ${message.id}``

This preserves the existing behavior when id is omitted while allowing an empty string to be serialized as an empty id field.

Test

Added a regression test using:

writeSSE({ data: 'reset', id: '' })

The test verifies the exact output:

data: reset
id:

Local result:

src/helper/streaming/sse.test.tsx
19 tests passed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.04%. Comparing base (cadff88) to head (58158cf).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5138   +/-   ##
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  Coverage   79.04%   79.04%           
=======================================
  Files         154      154           
  Lines       10779    10779           
  Branches     2256     2256           
=======================================
  Hits         8520     8520           
  Misses       2259     2259           

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