Dynamic schemas (settings, settings/lsp/*, debug_tasks) are now cached
after first generation and returned immediately via Task::ready() on
subsequent requests.

Cache is invalidated when `notify_schema_changed()` is called, which
happens when:
- Extensions are installed/uninstalled (affects settings schema)
- DAP registry changes (affects debug_tasks schema)

This eliminates repeated foreground thread blocking for the same dynamic
schema.

**Note:** This PR is based on #46823 and should be merged after it.

Release Notes:

- N/A

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Co-authored-by: Ben Kunkle <ben@zed.dev>