No completion if the cache has action name that are partially matched

Repro:
- Populate the cache with `open bbc webpage`, where the explainer will indentify "open" and "webpage" as subphrase that translate to "browser.openWebPage".
- Then completion will fail with `open` because it couldn't transform the action name with only the first part of the subphrase.

#1478 is a partial fix that stop the exception to kill all completion. But ideally,
1. we are able to partiall match "open" and determine that we will get "browser.openWebPage", based on the only possible transform with prefix `open`.
2. provide web site complete, either from "memory" or agent returned known web sites.

Start by reproducing the failure with a cache entry for `open bbc webpage`, then inspect the completion path and the partial fix in #1478. Done means a partial `open` match can resolve to `browser.openWebPage` without aborting completion, and website suggestions can come from memory or the agent's known sites.

Written by the indexing model from the issue text.