fix(backend): stop cross-file test-isolation leaks (MOCK_MODE + wifi timer)

[andrescera]

What

Fixes two pre-existing, latent test-isolation leaks that only surface in a
full bun run --filter backend test run under a local file-execution order
that differs from CI. On the affected ordering the suite reports 1921 pass /
9 fail; CI (a different readdir order) is green on the same commit. With this
change the full suite is 1930 pass / 0 fail in every order.

The 9 failures were: 8 in kiosk-rpc.test.ts and 1 in
wifi.procedure.test.ts. They trace to two independent leaked-state sources.

Why

bun test runs every file in ONE shared global object, so any file that leaves
process.env, a module singleton, or a pending timer dirty poisons every file
that runs after it.

Leak 1 — mock-feedback-broadcast.test.ts leaks MOCK_MODE (8 kiosk failures).
Two beforeAll blocks set process.env.MOCK_MODE = "true" and never restored
it. MOCK_MODE=true makes isDevelopment() true; combined with a leaked
mockState.initialized, shouldUseMocks() becomes true for later files. The
kiosk RPC procedures (system.procedure.ts) then take the mock branch —
resolveActiveKioskDeps() returns the in-memory mock harness instead of each
test's injected spy deps, and the !shouldUseMocks() && !isRealDevice() gate is
bypassed. So the "emulated" tests wrongly succeed, the "real" tests run against
the wrong deps (empty spies), and the "waits for cog-display" test times out.

Leak 2 — rpc-network.procedure.test.ts leaks a wifi-scan timer (1 wifi failure).
Its network.configure block calls updateNetif() in mock mode. The mock
ifconfig lists wlan0, so processIfconfigOutput schedules a detached
setTimeout(wifiUpdateDevices, 1000). That timer fires ~1s later — inside a
following mock-mode suite — and registers the mock adapter dc:a6:32:12:34:57
(id 0) into the process-wide wifiInterfacesByMacAddress registry. In
wifi.procedure.test.ts, macForDeviceId("0") then resolves to that leaked
adapter instead of the test's own seeded interface, so the held device lock is
missed and the handler returns success instead of DEVICE_BUSY.

How

• mock-feedback-broadcast.test.ts: snapshot MOCK_MODE once and restore it in
  both teardowns — the same save/restore idiom already used by PR fix(test): restore MOCK_MODE so mock suites do not leak dev mode across the bun suite #153 for the
  mmcli-mode-validation / modem-migration leaks.
• rpc-network.procedure.test.ts: in the network.configure afterAll, drain
  the pending wifiUpdateDevices timer (while mocks are still on) and evict the
  registered adapter before restoring the environment, so nothing leaks past the
  file.

No production code changed. No test weakened, skipped, or deleted.

How to verify

bun run --filter backend test    # 1930 pass / 0 fail

Run it a few times — the failures were deterministic per file order, so a green
result across runs is the proof. The 8 kiosk + 1 wifi cases all pass; nothing
else changes.

Risks

Low — test-only. The wifi fix adds a bounded (~1s) drain to one afterAll; it
polls for the adapter and exits as soon as it appears, then removes it. If the
timer never fires the loop is capped at 2s and the subsequent removeWifiInterface
is a harmless no-op.

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✨ Finishing Touches

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• Create PR with unit tests
• Commit unit tests in branch fix/mock-env-wifi-timer-isolation

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