Add WebDAV support to storage-cli client and improve test quality

[kathap]

Enable WebDAV/DAV provider support in StorageCliClient by adding 'dav' to STORAGE_CLI_TYPES and mapping legacy 'webdav' provider name to 'dav'.
Needs cloudfoundry/capi-release#634 to be merged first.

Test improvements:

• Add comprehensive tests for all legacy provider mappings (AzureRM, AWS, Google, aliyun, webdav)
• Add test for native 'dav' storage-cli type
• Use begin/ensure blocks to prevent tempfile leaks on test failures
• Add helper methods (write_config_file, stub_config_for_droplets) to reduce repetition
• Stub additional_flags in #run_cli tests for stability
• Fix test descriptions for accuracy

Thanks for contributing to cloud_controller_ng. To speed up the process of reviewing your pull request please provide us with:

• A short explanation of the proposed change:
  Add 'dav' to the list of supported storage-cli types and map the legacy 'webdav' provider name to 'dav', enabling CCNG to use storage-cli with WebDAV blobstores.

• An explanation of the use cases your change solves
  Enable WebDAV blobstore support:
  Allows Cloud Controller to use storage-cli with WebDAV blobstores (previously only Azure, AWS, Google, and Aliyun were supported). This completes the set of supported providers for CAPI's
  storage-cli adoption.

  Support legacy fog configurations:
  Enables operators to migrate existing CAPI deployments that use fog with 'webdav' provider to storage-cli without changing their manifest configuration. The legacy 'webdav' provider name is
  automatically mapped to the native 'dav' storage-cli type.

  Unblock CAPI migration from fog:
  Removes the last blocker for migrating CAPI blobstore operations from the fog gem to storage-cli

• Links to any other associated PRs
  Refactor DAV client, add missing operations storage-cli#70
  Enhance storage-cli webdav config capi-release#634

• I have reviewed the contributing guide

• I have viewed, signed, and submitted the Contributor License Agreement

• I have made this pull request to the main branch

• I have run all the unit tests using bundle exec rake

• I have run CF Acceptance Tests