test(frontend): cover backend exchange env gate

[cursor]

Motivation

Recent exchange-rate work restricted backend exchange mode to LOCAL/STAGING builds so BETA/PROD cannot be opted into backend pricing by a canister runtime flag. That build-time gate controls a broad pricing path, so a regression could accidentally route production users away from the full frontend provider path.

Risky behavior now covered:

• BACKEND_EXCHANGE_ENABLED is true for local and staging-shaped builds.
• BACKEND_EXCHANGE_ENABLED is false for beta and production-shaped builds.

Changes

• Added src/frontend/src/tests/env/exchange.env.spec.ts.
• The spec dynamically imports src/frontend/src/env/exchange.env.ts under mocked app environment constants to assert the backend exchange environment matrix directly.

These tests materially reduce regression risk by locking the build-time production/staging split without relying only on indirect service-level mocks.

Tests

• PATH="$HOME/.nvm/versions/node/v24.17.0/bin:$PATH" npm run prepare
• PATH="$HOME/.nvm/versions/node/v24.17.0/bin:$PATH" npm run test -- src/frontend/src/tests/env/exchange.env.spec.ts
• PATH="$HOME/.nvm/versions/node/v24.17.0/bin:$PATH" npm run format:file --file=src/frontend/src/tests/env/exchange.env.spec.ts
• PATH="$HOME/.nvm/versions/node/v24.17.0/bin:$PATH" npm run lint -- --max-warnings 0 src/frontend/src/tests/env/exchange.env.spec.ts
• PATH="$HOME/.nvm/versions/node/v24.17.0/bin:$PATH" npx eslint "src/frontend/src/tests/env/exchange.env.spec.ts" --max-warnings 0

Note: dependencies were absent in the runner, so npm ci was run first with Node 24 from nvm.