test: add non-demo heartbeat coverage

[Talchain]

Context

PR #35 skipped the demo heartbeat test because demo mode uses a short-circuit path that never emits heartbeats (see src/routes/v1/stream-enhanced.ts:201-231).

Goal

Add a minimal, non-demo test that exercises the real stream route with a minimal graph and validates that heartbeats are emitted.

Acceptance Criteria

• Test uses a minimal valid graph (e.g., 2-3 nodes)
• Test sets SSE_HEARTBEAT_MS to a short interval (e.g., 500ms)
• Test validates that at least one heartbeat event is received
• Test validates heartbeat event schema (stream.heartbeat.v1)
• Test runs in < 3 seconds

References

• PR fix(p1c-2): SSE stability — reduces failing test files vs baseline #35: fix(p1c-2): SSE stability — reduces failing test files vs baseline #35
• Demo short-circuit: src/routes/v1/stream-enhanced.ts:201-231
• Heartbeat logic: src/routes/v1/stream-enhanced.ts:293-302