test(api): behavioural cover for the raw-route session gate

[rkm1]

Why

requireSessionMiddleware — the single supported gate for the raw
ext.routes escape hatch — merged in #236 with exactly one test: that it
carries the CORE_GATE_MIDDLEWARE tag. Nothing asserted the 401 that is its
entire reason for existing, and nothing asserted that it resolves the session
through #237's per-request memo.

That second gap is why #236 shipped a semantic regression through a textually
clean rebase. The gate was written before the memo landed, so it resolved the
session itself; the only thing that says so is a resolution count, and there
was no test counting.

What the tests found

Writing them turned up two real defects, both fixed here:

1. A live 401→500 on current main. The gate had lost the defensive read of
env.SESSION_SECRET it originally shipped with. Some deployments expose env
as a proxy whose getter throws for an unbound key; the escaping TypeError
turns a route whose documented answer is 401 into a 500. Removing the
duplicate session resolution was right — removing this read along with it
was not. Restored.

2. A comment that claimed something untrue. It said resolving directly
"would hand the handler an unfrozen copy". It would not: the gate used its
session for the null check and discarded it. The actual second cost is that
the gate and the handler behind it would answer from two independent
resolutions, so a revocation landing between them decides the two
differently. Corrected, and now covered by a test asserting they share one
frozen object.

The tests

middleware.test.ts — five behavioural cases: 401 with the handler never
invoked; handler runs and its response passes through for a session; a GET is
gated too (unlike csrf's safe-method skip); fails closed on a missing
SESSION_SECRET; 401 rather than 500 on a throwing getter.

request-identity.test.ts — the gate joins the T2 chain guard, plus three
memo cases: it reuses a resolution the request already paid for, it and the
handler share one frozen session, and the no-context fallback still refuses
an anonymous request.

Verification

Red in both directions, not just green:

Gate body under test	Result
#236's original	the 3 memo assertions fail
current main	the throwing-getter case fails
this branch	119 passed across the 3 files

tsc --build (the repo's lint) clean. No public API surface change, so no
snapshot regeneration.

🤖 Generated with Claude Code