feat(live-atc): send the filed route so the clearance can grant it

The route reached the local engine already — genSID() takes it and throws it
away — but never reached the backend, so the clearance could only issue a SID.

The payload mapping moves to shared/utils so it can be tested at all: app/
composables are outside the test glob, and the fallback chains and string
coercion in it were entirely uncovered.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/shared/backendVariables.test.ts

@@ -0,0 +1,68 @@
+import { describe, it } from 'node:test'
+import assert from 'node:assert/strict'
+
+import { buildBackendVariables } from '~~/shared/utils/backendVariables'
+
+/** A VATSIM flight plan and the engine variables a session starts from. */
+const build = (
+  flightPlan: Record<string, any> = {},
+  vars: Record<string, any> = {},
+  atisRunway: string | null = null
+) =>
+  buildBackendVariables({
+    flightPlan: { callsign: 'DLH39A', dep: 'EDDF', arr: 'EDDM', ...flightPlan },
+    vars: { callsign: 'DLH39A', dest: 'EDDM', sid: 'BIBAX1N', ...vars },
+    atisRunway,
+  })
+
+describe('buildBackendVariables — the filed route', () => {
+  it('forwards the route so the clearance can grant it', () => {
+    // Without this the backend never sees the route and the clearance stops
+    // at the SID.
+    assert.equal(
+      build({ route: 'SULUS5S SULUS Y101 ARMUT' }).route,
+      'SULUS5S SULUS Y101 ARMUT'
+    )
+  })
+
+  it('sends an empty route for a flight that filed none', () => {
+    // Demo and manual flights. The backend keeps the flow defaults, which are
+    // the wording without a route.
+    assert.equal(build().route, '')
+  })
+
+  it('sends an empty route rather than dropping the key', () => {
+    // A missing key and an empty one are the same to the backend, but the key
+    // being present keeps the payload shape stable across flight sources.
+    assert.ok('route' in build())
+  })
+})
+
+describe('buildBackendVariables — flight plan mapping', () => {
+  it('prefers the engine callsign over the plan', () => {
+    assert.equal(build({ callsign: 'DLH1' }, { callsign: 'DLH39A' }).callsign, 'DLH39A')
+  })
+
+  it('falls back to the plan callsign when the engine has none', () => {
+    assert.equal(build({ callsign: 'DLH1' }, { callsign: '' }).callsign, 'DLH1')
+  })
+
+  it('uses the ATIS runway over the engine one', () => {
+    assert.equal(build({}, { runway: '07C' }, '25R').runway, '25R')
+  })
+
+  it('falls back to the engine runway when the ATIS gives none', () => {
+    assert.equal(build({}, { runway: '07C' }).runway, '07C')
+  })
+
+  it('sends numeric values as strings', () => {
+    const payload = build({}, { squawk: 2341, initial_altitude_ft: 6000, qnh_hpa: 1015 })
+    assert.equal(payload.squawk, '2341')
+    assert.equal(payload.initial_altitude, '6000')
+    assert.equal(payload.qnh, '1015')
+  })
+
+  it('leaves taxi_route out so the backend computes the real one', () => {
+    assert.ok(!('taxi_route' in build()))
+  })
+})