…y resets (#321)

Users on corporate networks with TLS inspection proxies
intermittently saw ConnectionResetError (10054) during 'a365 setup all'. The
failure was non-deterministic — different steps each run, restarting sometimes
helped.

Root cause: all token acquisition called 'az account get-access-token' as a
subprocess. The Azure CLI is Python-based and uses an HTTP connection pool that
the TLS proxy would reset mid-execution during longer commands.

Fix: replaced all subprocess token acquisition with direct MSAL.NET calls via
AuthenticationService. On Windows, MSAL uses WAM — an OS-level broker that
communicates over IPC, bypassing the proxy entirely. On macOS/Linux, MSAL uses
browser or device code flow.

Changes:
- GraphApiService and ArmApiService now acquire tokens via IAuthenticationService
  (MSAL) instead of spawning 'az account get-access-token' subprocesses
- AzCliHelper retains only login-hint resolution (az account show); token
  acquisition methods removed
- BotConfigurator reads tenantId from config instead of 'az account show'
- NetworkHelper centralises connection-reset detection and warning message
- RetryHelper accepts constructor-level defaults; injectable in tests to skip
  backoff delays
- AuthenticationService gains ResolveLoginHintFromCacheAsync as fallback when
  az CLI is not available
- Added tests for ResolveLoginHintFromCacheAsync and NetworkHelper

Add forceRefresh to Graph token flow; improve testability

Introduce a forceRefresh parameter to Microsoft Graph token acquisition, allowing explicit cache bypass and fresh token retrieval. Update IMicrosoftGraphTokenProvider and MicrosoftGraphTokenProvider to support this, and refactor GraphApiService to propagate forceRefresh throughout token and header acquisition. Update all call sites to use named arguments for clarity. Enhance ClientAppValidator and related tests to exercise token refresh logic. Improve test setup for GraphApiService and fix JWT Base64Url encoding/decoding in AuthenticationService and tests. These changes increase reliability and testability, especially for scenarios involving token revocation or conditional access.