Casks installed before `appcast` was removed from the DSL cannot be
managed (uninstall/upgrade/info) because loading the cached .rb
caskfile triggers `method_missing`, which calls `ofail` and sets
`Homebrew.failed = true`.

Add a `REMOVED_METHODS` set to the DSL class. When a removed method
is encountered, issue a warning (`opoo`) instead of an error (`ofail`)
so the cask still loads cleanly and operations like `uninstall` exit
with code 0.

Fixes Homebrew#21602

Move error handling from DSL method_missing (opoo/ofail) to
the loader level. Unknown DSL methods now raise NoMethodError,
which Cask#refresh converts to CaskInvalidError. This lets the
existing fallback infrastructure in named_args.rb handle it
gracefully for uninstall/upgrade operations.

Remove unreachable `if method` guard in DSL#method_missing
(Ruby's method_missing always receives a Symbol, which is
always truthy). Combine test assertions so the Homebrew.failed
check also verifies the exception is raised.

…files

When loading an installed caskfile that contains removed DSL methods,
wrap the resulting CaskInvalidError as CaskUnreadableError so the
existing fallback in named_args.rb can handle it gracefully. This is
gated on @from_installed_caskfile to avoid affecting normal cask
loading, following the same pattern as formulary.rb.

- respond_to_missing? now returns false to match method_missing raising
  NoMethodError (previously returned true unconditionally)
- Test CaskUnreadableError message includes the removed method name
- Add comment explaining rescue ordering dependency between cask.rb and
  cask_loader.rb