fix(client): make space after "data:" optional in SSE parser

[serhiizghama]

Problem

streamFlow (js/genkit/src/client/client.ts) strips a hardcoded "data: " (6 characters) from each SSE event before passing the rest to JSON.parse. Per the SSE spec, the single space after data: is optional — sources are allowed to emit data:foo. When they do (e.g. Spring WebFlux's built-in ServerSentEventHttpMessageWriter), the current parser consumes the first character of the payload and JSON.parse throws:

SyntaxError: Unexpected non-whitespace character after JSON at position 9

Fixes #3728.

Solution

Strip the data: prefix first, then strip at most one optional leading space — matching the SSE spec's "if value starts with a U+0020 SPACE character, remove it from value" rule.

Testing

Added tests/client_test.ts covering both data:{...} (no space) and data: {...} (conventional) event forms. Local run:

$ pnpm test
# tests 127
# pass 127
# fail 0

[gemini-code-assist]

Code Review

This pull request updates the SSE parser in client.ts to correctly handle optional spaces after the data: prefix per the SSE specification and adds corresponding unit tests. A review comment identifies a potential TypeError when calling hasOwnProperty on a null or non-object result from JSON.parse and suggests a guard clause for better robustness.

[gemini-code-assist]

The chunk variable resulting from JSON.parse(payload) could be null or a non-object type (e.g., if the payload is the string "null" or a number). Accessing hasOwnProperty on null will throw a TypeError. Adding a guard clause to ensure chunk is a non-null object improves robustness and prevents potential crashes on unexpected but valid JSON payloads.

      const chunk = JSON.parse(payload);
      if (!chunk || typeof chunk !== 'object') {
        throw new Error('unknown chunk format: ' + JSON.stringify(chunk));
      }
      if (chunk.hasOwnProperty('message')) {
        sendChunk(chunk.message);
      } else if (chunk.hasOwnProperty('result')) {