fix(stream): handle invalid JSON escape sequences in MessageStream

[xodn348]

Summary

Fixes #996

When the Anthropic API streams tool-use deltas containing invalid JSON escape sequences (e.g. \d, \p, \s from regex patterns), partialParse() throws an unhandled error that crashes the entire MessageStream. This is a runtime crash with no way for consumers to recover gracefully.

Change

Wrapped the partialParse(jsonBuf) call in #accumulateMessage with a try-catch that silently skips the delta on parse failure. The stream continues processing subsequent events, and the final complete JSON (which uses proper \\d escaping) parses correctly.

Before:

newContent.input = partialParse(jsonBuf);

After:

try {
  newContent.input = partialParse(jsonBuf);
} catch {
  // invalid escape sequences (e.g. \d, \p, \s) — skip this delta, stream continues
}

This is the minimal surgical fix — one file changed, 5 lines added, 1 removed.

Verification

• tsc --noEmit passes (EXIT_CODE=0)
• npm test passes (641 passed, 5 failed pre-existing Nock connection errors in ToolRunnerE2E unrelated to this change)