Enable CI PR builds for Bitbucket forked repositories (external providers)

Request

Support CI pipeline execution for pull requests originating from Bitbucket forked repositories, even when:

-The fork is owned by an external provider.

-The base repository is defined as the pipeline’s codebase.

Harness should correctly resolve and fetch PR refs for fork-based Bitbucket PRs and allow the pipeline to execute successfully.

-Fork-based PRs trigger CI pipelines reliably.

-Harness correctly resolves Bitbucket fork PR references and checks out the PR source code.

-PR metadata (fork repo, source branch, commit SHA, target branch) is made available to the pipeline.

-Pipeline execution does not fail due to PR ref resolution errors.

-Behavior is consistent with PRs originating from the same repository.