Support Account Name + Key authentication for Azure Blob Storage batch exports

[buildwithmalik]

Summary

Follows #41383. Add Account Name + Account Key as an alternative authentication method for Azure Blob Storage batch exports. Currently only Connection String auth is supported (#43977, #44696).

Why

Account + Key auth gives users more flexibility: easier key rotation without rebuilding connection strings, cleaner custom endpoint configuration, and aligns with how we'd eventually support Managed Identity.

Scope

Backend:

• Update AzureBlobIntegration to store auth_method in config and support account_name + account_key in sensitive_config
• Update AzureBlobConsumer to use BlobServiceClient(account_url, credential) when account key auth is used
• Update AzureBlobContainerTestStep to support validating account key auth

Frontend:

• Add auth type selector to AzureBlobSetupModal
• Conditionally show connection string OR account name + account key fields

Tests:

• Parameterize existing tests to cover both auth methods
• Test locally using Azurite emulator

Acceptance Criteria

• Can create Azure Blob export using Account Name + Key
• Connection String auth still works (no regression)
• Connection test validates Account + Key before saving
• Account key stored encrypted via Integration model
• Unit and integration tests cover both auth methods

References

• BlobServiceClient with AzureNamedKeyCredential
• Azurite emulator setup
• Storage account access keys

[buildwithmalik]

@tomasfarias Can you add the team/batch-exports and optionally good first issue labels for this one?