[FSMCPB-167858] Extend refresh-token samples with ERROR event handling

[VGio]

Summary

• Adds SHELL_EVENTS.ERROR handling to all three refresh-token samples (JavaScript, TypeScript, SAP UI5) as required by the fsm-shell documentation
• Each sample now demonstrates the three mandatory error-handling requirements:
  1. User notification — error messages are surfaced to the user via a visible warning area
  2. Retry delay — waits 5 seconds before re-emitting REQUIRE_AUTHENTICATION to avoid hammering the Shell
  3. Retry limit — stops after 3 retries with a "please reload" message, preventing excessive API calls that could result in extension deactivation

Changes

All three samples (shell-sdk.service.js/ts)

• Register SHELL_EVENTS.ERROR listener in init()
• Add errorSubject (BehaviorSubject) and subscribeToError() method

JS + TS samples (index.js / index.ts)

• Subscribe to subscribeToError() and display the message in a #error-info element
• Add amber warning styling (.error-message) to CSS
• Add #error-info div to HTML

UI5 sample (View1.controller.ts)

• Subscribe to subscribeToError() and bind the message to the existing /errorMessage model property

Tests

• Add Vitest unit tests (5 tests each) to the JS and TS samples covering: error emission, retry count messages, MAX_RETRIES cap, retry delay, and unsubscribe
• Add vitest dev dependency and "test" script to both package.json files

Test plan

• npm test passes in refresh-token-sample-js (5/5)
• npm test passes in refresh-token-sample-ts (5/5)
• npx tsc --noEmit passes in refresh-token-sample-ts
• npm run ts-typecheck passes in refresh-token-sample-ui5

Closes FSMCPB-167858

[cla-assistant]

All committers have signed the CLA.