fix: escape SSE-unsafe Unicode line separators in streamed payloads - #1433

json_utils.dumps() writes SSE data: payloads with ensure_ascii=False, so
U+2028 / U+2029 / U+0085 go on the wire as raw UTF-8. httpx.aiter_lines()
uses str.splitlines(), which treats those three as line breaks: one data:
line becomes several and json.loads() raises JSONDecodeError. Re-subscribing
sends the same snapshot, so the task stays unreadable over streaming.

This change escapes exactly those three characters as \uXXXX in dumps().
Escapes are ASCII, so the frame stays on one line, json.loads() restores the
original text, and CJK / emoji still pass through unescaped. dumps() is the
serializer used by rest_dispatcher.py, jsonrpc_dispatcher.py and the v0.3
REST adapter for streamed events, so all three streaming paths are covered.

Verification

• tests/utils/test_json_utils.py + tests/client/transports/: 130 passed
  (the 12 existing aiter_lines mocks are untouched and still pass).
• New end-to-end cases drive a real httpx.Response through parse_sse_stream
  for all three separators at chunk sizes 1 / 7 / 4096.
• Mutation check: reverting dumps() to upstream makes all 13 new assertions
  fail; restoring it makes 26/26 pass in the two touched test files.
• Call chain checked against current main 28f8a62f25. sse-starlette's own
  framing was not re-tested here (the issue reporter covered it).

Known limitation

The client parser still uses httpx.aiter_lines(), so a third-party A2A server
that emits these characters raw can still split a line. Fixing that means
swapping to aiter_text() plus a CR/LF-only decoder and migrating the existing
aiter_lines mocks across the transport tests. I left that out of this bug-fix
PR to keep the fix small; happy to follow up if maintainers prefer the
client-side approach instead.

Checklist

• Follow the CONTRIBUTING Guide
• PR title follows Conventional Commits (fix:)
• Tests added
• Formatting / lint / type checks: the repo's CI Lint Code Base
  (ruff check, ruff format --check, ty check) passes on this head.
  The formatter was not run locally because the ruff wheel would not
  download here; CI is the authoritative check.
• No docs change needed

Fixes #1432 🦕