ci: validate stacked pull requests

[parvahuja]

Stack 2 of 8 · depends on #195 · next: #184

Summary

Run CI for pull requests targeting any branch, rather than only main or master.

Why this is separate

Every later PR targets the branch below it. Without this one-line workflow change, those reviewable stacked PRs would not receive hosted CI.

No SDK or package behavior changes.

Validation

• actionlint
• hosted CI exercises Python 3.11–3.14, packaging, install smokes, integration tests, and conformance

_{Managed as a stacked change with GitHub Stacks.}

[parvahuja]

we likely don't need to merge this - but its gets CI to run on the rest of the PR stack, which we need for now