Always register built-in chat tools (drop when: gates) (#316478)

* Remove when: gates from built-in chat tools

Built-in tools should always be registered so the tool list is stable
per session. The when-clauses caused tools to register/unregister at
runtime (e.g. on task discovery, test runs, notebook open), which busts
the system-prompt cache mid-conversation. See #316187.

Tools touched:
- get_task_output, run_task: drop TasksAvailableContext
- runTests, testFailure: drop TestingContextKeys.has* gates
- rename, usages: drop config.chat.tools.*.enabled gates
- resolveDebugEventDetails: drop chatSessionHasDebugTools gate
- copilot_readNotebookCellOutput: drop userHasOpenedNotebook gate

Fixes #316187

* Remove usages and rename tool configuration from chat settings