ci: add workflow_dispatch trigger to functions workflow

[Ankitsinghsisodya]

Problem

The Functions CI workflow can only be triggered by a push to main or a pull_request. There is no way to manually rerun it from the GitHub Actions UI without making a new commit or reopening a PR.

Proposed Solution

Add a workflow_dispatch trigger so maintainers and contributors can manually run the full Functions workflow from the Actions tab at any time — useful for debugging flaky tests or verifying fixes without a dummy commit.

[Ankitsinghsisodya]

/assign