Call run-tests action from different repos

[dariaguy]

Description of change

Added support for run-tests action to be called from a repository, and added an option to override client image tag

Pull Request check-list

Please make sure to review and check all of these items:

• Do tests and lints pass with this change?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?

NOTE: these things are not required to open a PR and can be done
afterwards / while the PR is open.

[Copilot]

Pull request overview

This PR extends the run-tests composite GitHub Action so it can be invoked from other repositories and allows overriding the client-libs test image tag.

Changes:

• Added new inputs repository and ref to control which repository and ref are checked out before running tests.
• Added a client-libs-test-image input and wired it into the test environment as CLIENT_LIBS_TEST_IMAGE_TAG and CLIENT_LIBS_TEST_STACK_IMAGE_TAG, defaulting to the Redis version when not provided.
• Updated the Redis stack image selection logic so that CLIENT_LIBS_TEST_STACK_IMAGE_TAG is only overridden by the internal mapping or Redis version when it has not already been set (allowing explicit overrides).

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[petyaslavova]

LGTM.