feat: add plugin-directory support for playwright

[oshirohugo]

Summary

When a plugin lives in a subdirectory (e.g. monorepos using plugin-directory), the Playwright workflow failed to resolve the correct Grafana versions to test against. The e2e-version action always looked for src/plugin.json at the repo root, ignoring plugin-directory. The only workaround was to manually duplicate the Grafana version range via run-playwright-with-grafana-dependency, which is easy to forget and gets stale.

This PR eliminates the workaround by automatically reading grafanaDependency from the plugin's own src/plugin.json (inside plugin-directory) when neither an explicit grafana-dependency input nor a root-level src/plugin.json applies.

A secondary bug was also fixed: the "Install Playwright Browsers" step was missing working-directory, causing npx to run from the repo root (where no node_modules exists in a monorepo layout), which silently installed a different playwright version globally and downloaded the wrong Chromium revision.

Closes #553.

Changes

• playwright.yml — auto-read grafanaDependency from plugin-directory/src/plugin.json
  Added a Read Grafana dependency from plugin.json step that runs when plugin-directory != '.' and no explicit grafana-dependency is provided. It reads grafanaDependency from ${plugin-directory}/src/plugin.json using jq and passes it as a fallback to the e2e-version action. This makes the workflow self-contained for monorepo plugins without requiring any manual input.

• playwright.yml — fix missing working-directory on browser install step
  The Install Playwright Browsers step lacked working-directory: ${{ inputs.plugin-directory }}. When the plugin is in a subdirectory, npx ran from the repo root (no local node_modules), fell back to downloading the latest playwright from the registry, and installed the wrong Chromium revision — causing all E2E test jobs to fail with "Executable doesn't exist". Added the missing working-directory to match every other npm step in the job.

Tests

• We checked that the changes didn't cause any issue when running without providing the plugin directory:
  https://github.com/grafana/plugins-drone-to-gha/actions/runs/24903292302
  https://github.com/grafana/plugins-drone-to-gha/actions/runs/24902780309

• We checked that the changes worked when provinding the plugin directory:
  https://github.com/grafana/plugins-drone-to-gha/actions/runs/24901502734

[sunker]

Hi Hugo, I just came across this PR. I actually opened another PR last week that tackles the same issue but with a slightly different approach: #696. One advantage of that approach is that it keeps the plugin-action and plugin-ci-workflows APIs in sync.

It’s still a draft for now, as I’m waiting on the SLO team to assist with testing.

[oshirohugo]

@sunker tks for letting me know Erick. I will close this PR then :D.

[oshirohugo]

Closing this PR in favor of #696