fix(arborist): avoid crash when peer back-off detaches a node (#9770)

Installing a package whose peers form a cycle with an already-installed
optional peer could crash with `TypeError: Cannot read properties of
null (reading 'explain')` instead of resolving or reporting a real
conflict.
A minimal trigger: `vite@8.1.4` declares an optional peer on
`@vitejs/devtools`, `@vitejs/devtools` peers back on `vite`, so
installing vite and then adding devtools crashes.

The root cause is in `#loadPeerSet`.
While resolving a package's peer edge through the parent's edge, the
recursive `#nodeFromEdge` call can place a compatible peer that replaces
and detaches the current node from the tree mid-iteration.
The now-invalid edge then reached `#failPeerConflict`, whose
`#explainPeerConflict` calls `node.resolve(edge.name).explain()` on the
detached node.
`resolve()` returns `null` for a node no longer in the tree, so
`.explain()` threw.

A detached node has been superseded by a compatible peer, so there is no
real conflict to report.
The fix adds a guard that stops processing when the node has been
detached, right before `#failPeerConflict`, mirroring the existing
top-of-loop detachment check.
This lets the install complete by keeping the compatible peer that
replaced the node (for the reproduction, `@vitejs/devtools` backs off to
a version that satisfies vite's optional peer range) rather than
crashing or raising a spurious `ERESOLVE`.

## References

Fixes #5222
Closes #4787

Author: manzoorwanijk

workspaces/arborist/lib/arborist/build-ideal-tree.js

@@ -1639,6 +1639,12 @@ This is a one-time fix-up, please be patient...
             continue
           }
 
+          // The recursion above may replace node with a compatible peer, detaching it.
+          // A detached node has no real conflict to report, so stop instead of crashing on it.
+          if (!node.parent) {
+            break
+          }
+
           // problem
           this.#failPeerConflict(edge, parentEdge)
         }

workspaces/arborist/test/arborist/build-ideal-tree.js

@@ -4838,6 +4838,53 @@ t.test('add does not leave invalid peerOptional edges when save=false', async t
   }, 'lockfile-mutating add should reject instead of keeping an invalid peerOptional edge')
 })
 
+t.test('circular peer back-off does not crash when node is detached mid-resolution (#5222)', async t => {
+  // host (installed) has optional peer plugin@^1.0.0. Adding plugin resolves plugin@2.0.0,
+  // whose peer set replaces it with plugin@1.0.0 to satisfy host, detaching plugin@2 mid-loop.
+  // Previously the invalid edge then crashed #explainPeerConflict on the detached node.
+  const registry = createRegistry(t, false)
+
+  const hostPack = registry.packument({
+    name: 'host',
+    version: '1.0.0',
+    peerDependencies: { plugin: '^1.0.0' },
+    peerDependenciesMeta: { plugin: { optional: true } },
+  })
+  const hostManifest = registry.manifest({ name: 'host', packuments: [hostPack] })
+  await registry.package({ manifest: hostManifest, times: 2 })
+
+  const pluginPacks = [
+    registry.packument({ name: 'plugin', version: '1.0.0', peerDependencies: { host: '*' } }),
+    registry.packument({ name: 'plugin', version: '2.0.0', peerDependencies: { host: '*' } }),
+  ]
+  const pluginManifest = registry.manifest({ name: 'plugin', packuments: pluginPacks })
+  await registry.package({ manifest: pluginManifest, times: 2 })
+
+  const path = t.testdir({
+    'package.json': JSON.stringify({
+      name: 'test-5222',
+      version: '1.0.0',
+      devDependencies: { host: '^1.0.0' },
+    }),
+    node_modules: {
+      host: {
+        'package.json': JSON.stringify({
+          name: 'host',
+          version: '1.0.0',
+          peerDependencies: { plugin: '^1.0.0' },
+          peerDependenciesMeta: { plugin: { optional: true } },
+        }),
+      },
+    },
+  })
+
+  const arb = newArb(path)
+  const tree = await arb.buildIdealTree({ add: ['plugin'], saveType: 'dev' })
+
+  t.equal(tree.children.get('plugin').version, '1.0.0',
+    'backs off to plugin@1.0.0 to satisfy the optional peer instead of crashing')
+})
+
 t.test('peerOptional prefers existing tree node over registry fetch (#9249)', async t => {
   // Reproduction: ts-jest has peerOptional jest-util@"^29||^30".
   // @types/jest@28 → expect@28 → jest-util@28 placed at root first.