fix(graphql): ensure scalarsMap in buildSchemaOptions is respected

[ssut]

PR Checklist

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• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

The scalarsMap in buildSchemaOptions was not used even though it was defined.

This PR also fixes issues like #3204 by correctly merging the scalarsMap, ensuring that duplicate ScalarsTypeMap entries are ignored by their type. e.g.

buildSchemaOptions: {
  numberScalarMode: 'integer',
  scalarsMap: [
    {
      type: Date,
      scalar: GraphQLDateTime,
    },
  ],
}

What is the new behavior?

This PR ensures that the scalarsMap in buildSchemaOptions is respected and correctly merged with the scalarsMap from ScalarsExplorerService.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[kamilmysliwiec]

lgtm

[kamilmysliwiec]

Sounds like this PR might have introduced a regression #3244