Python: Add SSE keepalive interval to AG-UI FastAPI endpoint

[vaibhav-patel]

Fixes #6611.

Problem

add_agent_framework_fastapi_endpoint serves the AG-UI event stream as a bare
StreamingResponse(media_type="text/event-stream") with no application-level heartbeat.
During long silent gaps between agent events (slow server-side tools like OCR or
retrieval), idle-timeout proxies in front of the endpoint (Azure ingress, nginx,
serverless front doors) drop the healthy connection and the client sees a spurious
HTTP 500. The Connection: keep-alive header is TCP-level only and emits no application
data during an idle stream.

Fix

Wrap the event-stream generator so that when no upstream event is produced within a
configurable interval, a transport-level SSE keepalive comment (: keepalive) is written
to the wire. SSE comment lines are a protocol no-op that clients and parsers ignore, so
real events still flush immediately and in order while only idle gaps trigger a ping.

• New parameter keepalive_interval_seconds on add_agent_framework_fastapi_endpoint
  (default 15.0; pass None to disable). Non-positive values raise ValueError.
• The upstream generator is drained by a single dedicated task feeding an asyncio.Queue,
  keeping its entire lifecycle in one contextvars context. This is required by the agent
  run/telemetry pipeline, whose streaming cleanup resets ContextVar tokens that must be
  reset in the context that created them (racing individual pulls with asyncio.wait_for
  breaks that and was observed to raise "Token was created in a different Context"). Client
  disconnects cancel the producer so the upstream generator closes cleanly.
• No new dependency (kept StreamingResponse; did not pull in sse-starlette).

Tests

Added focused async tests in tests/ag_ui/test_endpoint.py: a keepalive is emitted during
an idle gap while real events still pass through in order; no keepalive when events flow
back-to-back; empty stream and upstream-error paths; and endpoint-level acceptance,
disable (None), and validation. Full test_endpoint.py (59) and the HTTP round-trip
suites pass; ruff and mypy are clean.

Note

The keepalive defaults on (15s) and emits only benign SSE comment lines during idle gaps.
If you'd prefer it opt-in, defaulting keepalive_interval_seconds to None is a one-line
change.

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[Copilot]

Pull request overview

This PR adds an application-level SSE keepalive mechanism to the Python AG-UI FastAPI endpoint so that long idle gaps in the event stream don’t get dropped by idle-timeout proxies (e.g., Azure ingress/nginx), while preserving event order and immediate flushing for real events.

Changes:

• Introduces an SSE keepalive wrapper (_with_sse_keepalive) that injects : keepalive comment frames during idle periods.
• Adds a new keepalive_interval_seconds parameter to add_agent_framework_fastapi_endpoint (default 15.0, None disables; non-positive rejected).
• Adds unit/acceptance tests covering wrapper behavior and endpoint configuration/validation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
python/packages/ag-ui/agent_framework_ag_ui/_endpoint.py	Adds keepalive wrapper and wires it into the StreamingResponse pipeline via a new endpoint parameter.
python/packages/ag-ui/tests/ag_ui/test_endpoint.py	Adds tests for keepalive wrapper behavior plus endpoint-level enable/disable/validation checks.