Improve unit coverage and harden background/content-script flows

[bartekplus]

Summary

This PR improves test depth and reliability across background/content-script flows, adds missing unit tests for uncovered modules, and includes minor correctness fixes discovered while increasing coverage.

Key Changes

• Added focused unit suites for previously under-tested modules:
  • tests/error.test.ts
  • tests/variables.test.ts
  • tests/SpacingRulesHandler.test.ts
  • tests/TemplateExpander.test.ts
  • tests/TextExpansionManager.test.ts
  • tests/PresageEngine.test.ts
  • tests/Migration.test.ts
  • tests/utils.additional.test.ts
• Added high-value routing/behavior tests for main orchestration files:
  • tests/background.routing.test.ts
  • tests/content_script.behavior.test.ts
• Included related updates in existing tests:
  • tests/content_script.watchdog.test.ts
  • tests/utils.test.ts
  • tests/e2e/puppeteer-extension.test.ts
• Source fixes:
  • src/background/SpacingRulesHandler.ts
    • Fix spacing behavior when trailing insertion is disabled but preceding space must still be removed.
  • src/background/background.ts
    • Fix async error handling for migrateToLocalStore(...) in onInstalled update flow (.catch(...) instead of sync try/catch).

Coverage Impact

• Unit coverage significantly improved overall.
• High line coverage was achieved for key orchestration files:
  • src/background/background.ts
  • src/content-script/content_script.ts

Validation

Executed successfully:

• npm run check
• npm test
• npm test -- --coverage
• npm run test:e2e

Notes

• This PR intentionally includes all currently modified files from the working tree, per request.