fix: harden SSE transport and enforce request body size limit - #1184

What changed

1. Configure SSE heartbeat and stream timeout explicitly

Problem: EventSourceResponse was created without explicit ping/send_timeout in src/a2a/server/routes/jsonrpc_dispatcher.py (_create_response) and src/a2a/server/routes/rest_dispatcher.py (_handle_streaming). With the installed sse-starlette, the default ping is None — no heartbeat is emitted — so idle streams send nothing and dead clients are never detected; there is also no cap on how long a stream may sit without producing output.

Fix (src/a2a/server/routes/jsonrpc_dispatcher.py, src/a2a/server/routes/rest_dispatcher.py):

• Added SSE_PING_INTERVAL_SECONDS = 15 and SSE_SEND_TIMEOUT_SECONDS = 300 to src/a2a/utils/constants.py.
• Every EventSourceResponse created by both dispatchers now passes ping=15 (heartbeat frame every 15s) and send_timeout=300 (tear down a stream that makes no progress for 5 minutes).

2. Enforce a request body size limit

Problem: No limit was enforced on the HTTP request body. A client could POST an arbitrarily large payload, which the JSON-RPC (request.json()) and REST (request.body()) paths buffer fully in memory — a memory-exhaustion / DoS vector.

Fix (src/a2a/server/routes/common.py, src/a2a/server/routes/jsonrpc_dispatcher.py, src/a2a/server/routes/rest_dispatcher.py):

• Added MAX_REQUEST_BODY_SIZE = 10 MiB to src/a2a/utils/constants.py.
• Added read_request_body_with_limit() in routes/common.py: it fast-rejects via the Content-Length header when present, and otherwise streams the body in chunks with an incremental cap, so oversized chunked bodies are rejected while being read rather than buffered unboundedly. The body is cached on the request so existing request.body()/stream() consumers keep working.
• JSON-RPC: handle_requests now reads through the limited reader; the existing HTTPException(413) handling returns InvalidRequestError("Payload too large").
• REST: all four body-reading sites (_handle_streaming pre-consume, on_message_send, on_message_send_stream, set_push_notification) now use a wrapper that converts the 413 into InvalidRequestError("Payload too large") (400).

Testing

• ./.venv/Scripts/python -m pytest tests/server/routes/test_jsonrpc_dispatcher.py tests/server/routes/test_rest_dispatcher.py -q → 39 passed (includes new tests: oversized body rejected via body content and via Content-Length, and ping_interval/send_timeout assertions on the returned EventSourceResponse for both dispatchers).
• ./.venv/Scripts/python -m pytest tests/server/ tests/compat/ -q → 924 passed, 90 skipped, 3 xfailed (pre-existing xfails reference upstream issue [Feat]: Improve server concurrency architecture #869).
• ./.venv/Scripts/python -m ruff check on all modified files: clean.
• Behavior change: SSE streams now emit a heartbeat comment every 15s (previously none with the installed sse-starlette) and are torn down after 300s without progress; request bodies over 10 MiB are rejected with 413/"Payload too large" instead of being buffered.