fix(db-mongodb): handle admin.condition in sparse unique indexes

[rachit367]

What?

formatBaseSchema in packages/db-mongodb/src/models/buildSchema.ts decides whether a unique index should be sparse by checking the static field.required property. A field with required: true always gets a non-sparse unique index, even when admin.condition makes it only conditionally required.

This change treats a field as effectively optional for index purposes whenever required: true is combined with an admin.condition, so a sparse unique index is created and documents that omit the field do not collide on null.

Why?

Fixes #16298.

Today, the following field definition creates a non-sparse unique index:

{
  name: 'slug',
  type: 'text',
  unique: true,
  required: true,
  admin: {
    condition: (data) => data.type === 'page',
  },
}

Two documents where type !== 'page' both leave slug as null, and the second insert fails with a Mongo unique constraint violation because the index treats both nulls as duplicates.

How?

buildSchema.ts:

• Introduce effectivelyOptional = field.required !== true || Boolean(field.admin?.condition).
• Use it in place of the existing field.required !== true check inside the sparse-index branch.

No behavior change for fields without admin.condition — the expression collapses to the original check. Fields that are conditionally required now get sparse: true, matching what already happens for required: false, localized, or drafts-enabled fields.

Scope is limited to @payloadcms/db-mongodb since the issue is reported against the Mongo index builder; other adapters are not touched.