Add a validation if the `service-dir` existing path is different than configed one

In https://github.com/Azure/azure-rest-api-specs/pull/34078, we found that when the existing path does not match the path configured in tspconfig.yaml , CI error occurs.
To avoid similar errors, we will add a validation for `service-dir`.

Start by locating the service-dir path handling and the tspconfig.yaml validation entry point, then compare the configured and existing paths using the scenario from PR #34078. Done means a mismatch is reported during validation before CI reaches the later error.

Written by the indexing model from the issue text.