[FEATURE] Add test datasource connection button

[adrianSepiol]

…Button for testing datasource connections

Related pull requests (in order):

1. [BUGFIX] set correct ContentLength for request body perses#4007
2. This one
3. [FEATURE] integrate test datasource connection button into DatasourceDrawer perses#4008
4. [BREAKINGCHANGE] Replace local buildProxyUrl with shared implementation from @perses-dev/client perses#4302
5. [FEATURE] add test connection button to prometheus plugin plugins#620

Related issue:
perses/perses#1542

Description

Adds a Test Connection button to the datasource editor that validates connectivity before saving. The button is wired through the editor component chain (DatasourceEditorForm → PluginEditor → PluginSpecEditor) where the connection logic is bound to the plugin's healthCheckPath and the current project/dashboard context. As part of this, buildProxyUrl is also migrated to @perses-dev/client, replacing the local implementation in perses/ui/app that will be removed in a companion PR.

Screenshots

In perses/plugins#620

Checklist

• Pull request has a descriptive title and context useful to a reviewer.
• Pull request title follows the [<catalog_entry>] <commit message> naming convention using one of the
  following catalog_entry values: FEATURE, ENHANCEMENT, BUGFIX, BREAKINGCHANGE, DOC,IGNORE.
• All commits have DCO signoffs.

UI Changes

• Changes that impact the UI include screenshots and/or screencasts of the relevant changes.
• Code follows the UI guidelines.
• E2E tests are stable and unlikely to be flaky.
  See e2e docs for more details. Common issues include:
  • Is the data inconsistent? You need to mock API requests.
  • Does the time change? You need to use consistent time values or mock time utilities.
  • Does it have loading states? You need to wait for loading to complete.

[jgbernalp]

LGTM, sorry for the delay. As a follow up we can add the test button to the global datasources as well

[jgbernalp]

@adrianSepiol might need a rebase