[Bug] Save data migrator to fix starters with no selectable forms

[Wlowscha]

What are the changes the user will see?

Players who previously could select E-Max Eternatus or other forms supposed not to be selectable from the starter screen will now see the default form, as intended.

Why am I making these changes?

After #5119, catching a battle form did not automatically unlock the default form. This caused unintended behavior in the starter select screen, where the caught battle form would show up and be selectable, most noticeable for players who caught Eternatus in its E-Max form. The bug was fixed in #5385, and applies to Pokémon caught from 1.7.0 onwards, but the issue still exists for already problematic save data. In order to fix this, a save data migration is necessary.

What are the changes from a developer perspective?

A new migrator function is added to systemMigrators. Its purpose is to scan data.dexData to find starters which are caught, but for which only unselectable forms are marked as caught. For any such starters, the default form is automatically marked as caught.

Screenshots/Videos

How to test the changes?

Checklist

• I'm using beta as my base branch
• There is no overlap with another PR?
• The PR is self-contained and cannot be split into smaller PRs?
• Have I provided a clear explanation of the changes?
• Have I tested the changes manually?
• Are all unit tests still passing? (npm run test)
  • Have I created new automated tests (npm run create-test) or updated existing tests related to the PR's changes?
• Have I provided screenshots/videos of the changes (if applicable)?
  • Have I made sure that any UI change works for both UI themes (default and legacy)?

[Snailman11]

https://discord.com/channels/1125469663833370665/1338179743102668983

[Wlowscha]

This was tested on a save file with three caught forms: E-Max Eternatus, Rain Castform, and W Unown. This is what the starter select screen looks like in a previous version:

After loading up the same save with the code in this PR, it looks like this:

To test other forms, generate a new savefile. This can be done by using overrides such as

const overrides = {
  STARTING_WAVE_OVERRIDE: 11,
  OPP_SPECIES_OVERRIDE: Species.ETERNATUS,
  OPP_FORM_OVERRIDES: {[Species.ETERNATUS]: 1},
  POKEBALL_OVERRIDE: {
    active: true,
    pokeballs: {
      [PokeballType.POKEBALL]: 0,
      [PokeballType.GREAT_BALL]: 0,
      [PokeballType.ULTRA_BALL]: 0,
      [PokeballType.ROGUE_BALL]: 0,
      [PokeballType.MASTER_BALL]: 99,
    },
  }
} satisfies Partial<InstanceType<typeof DefaultOverrides>>;

on a version of the code pre- #5385 . It is necessary to have a full party when catching the form, and refuse when asked to add it to the party.