ci: add workflow_dispatch trigger so fork branches can run CI before a PR exists #1116

Expected Behavior

.github/workflows/ci.yml gains a workflow_dispatch trigger, so a contributor can run the build against a branch in their own fork before opening a pull request:

on:
  pull_request: {}
  workflow_dispatch: {}

A push trigger scoped to main would additionally give post-merge signal, but workflow_dispatch alone solves the contributor-facing problem.

Current Behavior

CI triggers on pull_request only:

on:
  pull_request: {}

A fork branch cannot be built by Actions at all until a PR exists against this repository, and once one exists there is no way to re-run the workflow from the Actions tab on demand.

Context

Preparing three PRs against the streamable HTTP transports, I could not get CI feedback on my own fork before opening them. The two workarounds both cost the maintainers something:

• Open the PR early as a draft purely to borrow CI, which puts unfinished work on the maintainers' review queue.
• Push an empty commit to force a re-run when a job fails for an environmental reason. I have one of these in my own branch history (941b3540, "chore(ci): retrigger build") that exists for no other purpose and is now permanently in the commit log of a PR.

This matters more than usual for this repo because a meaningful slice of the suite is Testcontainers-based and does not run without local Docker, so "just run ./mvnw verify locally" is not a complete substitute for CI on every machine.

I'm happy to open the PR if the change is wanted.

This issue was created with AI assistance.