[18.0][FIX] base_substate: missing space in substate validation message

[bosd]

The substate validation error currently reads:

The substate Gevalideerd is not defined forthe state Done but for Te factureren

Problem

The message is built with f-strings inside self.env._():

self.env._(
    f"The substate {rec.substate_id.name} is not defined for"
    f"the state {rec_states[rec.state]} but for "
    f"{rec_states[target_state]}"
)

Two consequences:

• "... is not defined for" and "the state ..." are concatenated with no
  space in between → forthe;
• the msgid contains the interpolated values, so it can never be extracted
  nor matched against a .po entry. The string is indeed absent from
  i18n/base_substate.pot, and the message is always rendered in English
  regardless of the user language.

Change

Restore the named placeholders that were used up to 17.0. This fixes the
missing space and makes the message translatable again — as a bonus the
existing es / it translations for this msgid apply once more.

The trailing space at the end of the msgid is kept deliberately: it is what
es.po / it.po already carry, so keeping it preserves those translations.

No functional change beyond the message text.