Fix: Merge type selections for inline fragments with shared field names

[morticue]

• Added automated tests
• Documented for all relevant versions
• Updated the changelog

Changes

Fix codegen skipping type classes in inline fragments with shared field names

The codegen was only generating the first type class when multiple inline
fragments selected the same field name but returned different concrete types.

For example, given this query:

... on NotePublishedCompanyUpdateChange {
    article { pushNotificationBody }
}
... on ResearchUpdatePublishedCompanyUpdateChange {
    article { pushNotificationBody }
}
... on InitiationCoveragePublishedCompanyUpdateChange {
    article { pushNotificationBody }
}

Where each article field resolves to a different concrete type (Note, Update,
InitiationCoverage), only the Note.php class would get generated. The other two
were skipped, even though the generated code still referenced them in the type
converters. This obviously broke static analysis and caused runtime errors.

The problem was in OperationStack::setSelection(). It used ??= which meant
"only set if not already set":

$this->selections[$namespace] ??= $selection;

So when processing the fragments:

• First one sets selections for Note - works fine
• Second one tries to add Update, but selections[$namespace] already exists,
  so it gets ignored
• Same thing happens with InitiationCoverage

Changed it to merge selections by adding only new types that haven't been
processed yet:

if (!isset($this->selections[$namespace])) {
    $this->selections[$namespace] = $selection;
} else {
    // Only add new types, don't overwrite existing ones
    // (we already processed that type's subtree)
    foreach ($selection as $typeName => $builder) {
        if (!isset($this->selections[$namespace][$typeName])) {
            $this->selections[$namespace][$typeName] = $builder;
        }
    }
}

This selective merging ensures all types get generated while avoiding
reprocessing of types that were already handled (which would lose their
field selections).

This only affects schemas where you have inline fragments on interfaces with
fields that return different concrete types per implementation. Union types
already worked because they handle all possible types in a single pass.->

Breaking changes

[spawnia]

Could you please merge the latest master into your branch? I tried to do it but don't have push access to your fork.

[morticue]

Merged in the master branch now 👍