Ensure that a search with an immediately canceled token does not actually start a search. Fix possibly flaky findFiles test

Fix #57418 - Assert that totalChars > leadingChars in preview options. And truncate match when it's > totalChars

Fix #57046 - workspaceContains should search using search providers, not its own DiskSearch instance

There are many places in the code that do `import Event, { ...} from '.../event'`.

Update src to build with TS 2.7

Explicitly assign this.message in the else body