increase unit test coverage for terraform plugin framework external client implementation

https://github.com/crossplane/upjet/pull/329 is slightly below the code coverage diff and needs to be improved.
This PR is extensively tested with e2e tests.

Start by reviewing pull request #329 and the Terraform plugin framework external client implementation. Identify the unit-test coverage gap behind the coverage diff, then add focused unit tests alongside the existing tests and verify that the coverage threshold is met; the payload does not name specific files or test commands.

Written by the indexing model from the issue text.