PromptsService: add caching to listPromptFiles, make it consstent with caching of slash commands and custom agents (#277258)

* PromptsService: add caching to listPromptFiles, make it consstent with caching of slash commands and custom agents

* Update src/vs/workbench/contrib/chat/common/promptSyntax/service/promptsServiceImpl.ts

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update src/vs/workbench/contrib/chat/common/promptSyntax/service/promptsServiceImpl.ts

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update src/vs/workbench/contrib/chat/common/promptSyntax/service/promptsService.ts

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* polish

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>