azure: Use BearerChallengePolicy from auth instead of local duplicate (#2232)

* azure: Use BearerChallengePolicy from auth instead of local duplicate

Replace AzExtBearerChallengePolicy and getDefaultScopeFromEndpoint in the
azure package with imports from @microsoft/vscode-azext-azureauth. The
telemetry properties (challenge, challengeSuccess) are preserved by wrapping
the callback at the call site with try/catch for robustness.

This adds @microsoft/vscode-azext-azureauth as a dependency of the azure
package, consolidating the challenge policy logic in one place.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address PR feedback: rethrow errors, log details, extract helper

- Extract createBearerChallengePolicy() helper to deduplicate policy
  creation between createAzureClient and createAzureSubscriptionClient
- Rethrow errors from challenge handler instead of swallowing them
- Log parseError(err).message to telemetry as challengeError on failure

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* azure: Install @microsoft/vscode-azext-azureauth@6.0.0-alpha.6

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: alexweininger