Hand an aborted leader's fetch to one follower instead of all of them [patch]

[matt-edmondson]

Fixes #58

What

If a leader's client disconnected mid-transfer, Ticket.Dispose released its followers with false. Each follower then fetched the object from upstream on its own, so 1 leader and 5 followers produced 6 upstream fetches.

This implements the issue's first option.

• ObjectRouteHandler.DownloadAsync: a follower released without the object (an aborted or failed leader, or a FollowerTimeout) disposes its ticket and calls coalescer.Acquire again. One released follower becomes the new leader and the others wait for it. The loop is bounded by MaxFollowerAttempts = 3; after that a follower fetches for itself, as it did before, so a run of failing leaders can't hold a request forever. The coalesced_waits metric is still recorded once per request.
• SingleFlight: Complete and Dispose now retire the entry before they set its result. Before, a released follower that re-acquired quickly could find the finished entry still registered and get a follower ticket whose result was already false. That wasted one of its attempts.

I did not do the second option, which runs the leader's fetch on a token that isn't tied to its client. It would mean detaching the upstream fetch-and-store from the response stream the leader is writing. That is a larger change, and this fix already meets the acceptance criterion without it.

Tests

• ProxyFlowTests.AbortedLeader_HandsTheFetchToOneFollowerInsteadOfEveryFollowerFetching is the issue's repro. It holds the upstream behind a gate, queues 5 followers behind a leader, cancels the leader's request and then opens the gate. It asserts that every follower gets the right bytes and that upstream saw exactly 2 fetches. A coalesced_waits meter listener confirms the followers are queued before the abort.
• StubUpstream has a new ObjectGate that holds object fetches in flight. A fetch whose request is cancelled while it waits is abandoned.

Verification

• The new test fails on unchanged main at Assert.AreEqual(2, FetchCount), and passes with the fix.
• The full suite passes: 355 of 355.
• The new test passed 8 runs in a row, alongside ConcurrentMissesForOneObject_ProduceASingleUpstreamFetch.

#107 also edits ObjectRouteHandler, but in the store-hit path, so any conflict should be small.

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https://claude.ai/code/session_017ft9gJVQx6JFNQ4VRoAwd9

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[sonarqubecloud]

Quality Gate passed

Issues
4 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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