dtutils.string: build_substitute_list errors when a metadata field (e.g. rights) has been removed

Describe the bug

lib/dtutils/string.lua build_substitute_list() throws when a metadata field it references has been removed from darktable's metadata definitions. Any script using variable substitution (e.g. contrib/rename_images) then fails, regardless of which variables its pattern actually uses.

In darktable 5.x, metadata fields are user-configurable (stored in data.db meta_data). If a user removes e.g. Xmp.dc.rights, the rights field is no longer registered on dt_lua_image_t, and indexing it raises an error rather than returning nil. So the existing guard

image.rights and image.rights or "",   -- RIGHTS

doesn't protect against it. The same applies to image.title, image.description, image.creator, image.publisher and image.version_name (lines ~810 and 877–886).

To reproduce

1. In the metadata editor preferences, remove the rights (Xmp.dc.rights) field and restart darktable.
2. Enable contrib/rename_images.
3. Rename any image with any pattern, e.g. $(EXIF.YEAR)$(EXIF.MONTH)$(EXIF.DAY)_$(SEQUENCE).$(FILE_EXTENSION).

Log (darktable -d lua)

LUA pattern is $(EXIF.YEAR)$(EXIF.MONTH)$(EXIF.DAY)_$(SEQUENCE).$(FILE_EXTENSION)
LUA ERROR : ...urces/share/darktable/lua-scripts/lib/dtutils/string.lua:885: field "rights" not found for type dt_lua_image_t

stack traceback:
	[C]: in ?
	[C]: in metamethod 'index'
	...urces/share/darktable/lua-scripts/lib/dtutils/string.lua:885: in function 'lib/dtutils.string.build_substitute_list'
	...es/share/darktable/lua-scripts/contrib/rename_images.lua:144: in upvalue 'do_rename'
	...es/share/darktable/lua-scripts/contrib/rename_images.lua:211: in function <...es/share/darktable/lua-scripts/contrib/rename_images.lua:210>
	[C]: in ?

Suggested fix

Read the metadata fields through a protected lookup, e.g.

local function get_field(image, field)
  local ok, value = pcall(function() return image[field] end)
  return (ok and value) and value or ""
end

and use get_field(image, "rights") etc. in the replacements table.

Workaround

Re-add the missing metadata field in the metadata editor preferences.

Platform

• darktable 5.6.0 (Lua API 9.7.0), lua-scripts as bundled with the official macOS package
• macOS 26.6.2, Apple M2

Start in lib/dtutils/string.lua at build_substitute_list(), then inspect the call from contrib/rename_images.lua. Reproduce with darktable -d lua after removing the rights metadata field, and verify that variable substitution and image renaming work without the field being registered.