fix: handle escape sequences

[vvaswani]

Fixes #882

[MaxwellCalkin]

I ran into this same issue when testing MCP tool results containing Unicode line separators. Confirmed this fix is correct — JSON.stringify does not escape U+2028/U+2029, but Node's readline treats them as line terminators, splitting the JSON output mid-line.

Verified that the only call site needing the fix is toReadableStream() in src/core/streaming.ts. The replacement approach here (post-stringify escaping to \u2028/\u2029) is clean and round-trips correctly through JSON.parse.

Also tested U+0085 (NEL) — Node's readline does NOT treat it as a line separator, so no additional escaping is needed. This PR's scope is correct.

Would be great to get this merged — it's been open since January 30 and the fix is minimal (5 lines).