test: cover the two locations the composition test was carrying

Moving `web-agent-presets` to the built lane took its coverage with it, and the
per-file gate named what it had been the only reach for: the session header's
`agentPreset` type check, and the preset subtree's `write()` override. Neither
belonged to a real-composition boot — a header validation case and a subtree
behaviour are package-level facts.

The `write()` test states why the override exists: the inherited method
persists the tree whenever the Loader thinks the config moved, and disposing an
agent disposes its subtree, so inheriting it truncates the shipped preset to
`[]` the first time a session ends.

packages/core/session/tests/session.spec.ts

@@ -1319,6 +1319,7 @@ describe('SessionStore', () => {
       { meta: { delegationDepth: '1' }, error: /delegationDepth must be a non-negative safe integer/ },
       { meta: { delegationDepth: 0.5 }, error: /delegationDepth must be a non-negative safe integer/ },
       { meta: { delegationDepth: -1 }, error: /delegationDepth must be a non-negative safe integer/ },
+      { meta: { agentPreset: 1 }, error: /agentPreset must be a string/ },
     ]
 
     for (const [index, { meta, error }] of cases.entries()) {