fix: keep fleet prune estimate bytes after a target timeout (#1783)

* fix: keep fleet prune estimate bytes after a target timeout

Accumulate successful per-target reclaimable bytes on both the local
serial loop and the remote fan-out, mark partial nodes, and surface that
state in the Fleet prune card instead of zeroing the whole node.

* test: clarify zero-byte partial estimate card case

Rename the FleetPruneCard assertion so it matches fold semantics:
partial with zero bytes means a successful zero-byte target plus a failure,
not an all-failed node.

* fix: keep fleet prune estimate resilient on init and partial errors

Guard DockerController init so a deleted-node race cannot 500 the whole
fleet estimate, surface partial-node failure text in the card row title,
and pin the generic-rejection partial path with a fast route test.

* test: destroy reverse-route connect-ack sockets on teardown

Unguarded accepted sockets could emit a late ECONNRESET after assertions passed, failing the Backend Vitest job with zero assertion failures.

backend/src/__tests__/fleet-estimate.test.ts

@@ -0,0 +1,58 @@
+import { describe, it, expect } from 'vitest';
+import { foldNodeEstimate } from '../helpers/fleetEstimate';
+
+const node = { nodeId: 1, nodeName: 'local' };
+
+describe('foldNodeEstimate', () => {
+  it('returns the sum when every target succeeds', () => {
+    expect(foldNodeEstimate(node, [
+      { bytes: 500 },
+      { bytes: 100 },
+    ])).toEqual({
+      nodeId: 1,
+      nodeName: 'local',
+      reclaimableBytes: 600,
+      reachable: true,
+    });
+  });
+
+  it('keeps successful bytes and marks partial when some targets fail', () => {
+    expect(foldNodeEstimate(node, [
+      { bytes: 500 },
+      { bytes: 0, error: 'Docker daemon is busy. Please try again in a moment.' },
+      { bytes: 100 },
+    ])).toEqual({
+      nodeId: 1,
+      nodeName: 'local',
+      reclaimableBytes: 600,
+      reachable: true,
+      partial: true,
+      error: 'Docker daemon is busy. Please try again in a moment.',
+    });
+  });
+
+  it('marks the node unreachable when every target fails', () => {
+    expect(foldNodeEstimate(node, [
+      { bytes: 0, error: 'first failure' },
+      { bytes: 0, error: 'second failure' },
+    ])).toEqual({
+      nodeId: 1,
+      nodeName: 'local',
+      reclaimableBytes: 0,
+      reachable: false,
+      error: 'first failure',
+    });
+  });
+
+  it('surfaces the single-target failure message without partial', () => {
+    expect(foldNodeEstimate(node, [
+      { bytes: 0, error: 'Docker daemon is busy. Please try again in a moment.' },
+    ])).toEqual({
+      nodeId: 1,
+      nodeName: 'local',
+      reclaimableBytes: 0,
+      reachable: false,
+      error: 'Docker daemon is busy. Please try again in a moment.',
+    });
+  });
+});