test(pi): let a run finish and the actor sleep when its caller disconnects

[eersnington]

Tests for a caller that gives up on a long prompt, for both pi() and piDurable().

client: prompt("tell a story") over HTTP ── abort while the model answers
actor:  the run keeps going ── answer stored ── idle ── sleeps

• The run finishes and its answer is stored.
• The actor sleeps only after the answer is done, not in the middle of it.
• Needs fix(rivetkit): stop hibernating actor connections by default rivet#5835. Without it, the aborted request's connection stays registered and the actor never sleeps.

---

This is part 6 of 11 in a stack:

•  11  fix(pi): persist migrated pi() session entries #330
•  10  feat(pi): expose the pi session to actor code as c.pi #329
•  9  perf(pi): connect the pi() sandbox on first use #328
•  8  feat(pi): drain and resume pi() runs #327
•  7  chore(examples): bump the docs examples to pi-coding-agent 1.0.2 #326
•  6  test(pi): let a run finish and the actor sleep when its caller disconnects #325 👈
•  5  feat(pi): drain pi durable actors across upgrades #324
•  4  feat(pi): stream pi durable conversations to clients #323
•  3  feat(pi): add piDurable actor #322
•  2  feat(pi): store pi durable sessions in actor sqlite #321
•  1  feat(pi): add piDurable() and drain, resume, and c.pi for pi() #320