chore(js/core): Include key for all actions

[ifielker]

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

[gemini-code-assist]

Code Review

This pull request introduces a standardized key property for actions, formatted as /${type}/${name}, and ensures it is populated during registration and included in telemetry metadata. The changes update actionMetadata creation, the registry's action registration logic, and various tests to verify the genkit:key attribute in exported spans. One review comment suggests that registerActionAsync also needs to be updated to ensure consistency when namespaces are used, as it currently might report non-namespaced keys for actions registered asynchronously.