fix: make platformCustomerId optional in Kotlin sample (#348)

## Summary
- Update Kotlin sample to treat `platformCustomerId` as optional,
syncing with schema change from #345
- Remove unused `requireText` import

## Context
The schema change in #345 made `platformCustomerId` optional on customer
creation (if not provided, one is auto-generated). The Mintlify docs
were updated in that same PR, but the Kotlin sample still used
`requireText()` which would throw an error if the field was missing.

## Test plan
- [ ] Verify Kotlin sample compiles
- [ ] Test customer creation without `platformCustomerId` in request
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samples/kotlin/src/main/kotlin/com/grid/sample/routes/Customers.kt

@@ -9,7 +9,6 @@ import com.grid.sample.GridClientBuilder
 import com.grid.sample.JsonUtils
 import com.grid.sample.Log
 import com.grid.sample.optText
-import com.grid.sample.requireText
 import io.ktor.http.*
 import io.ktor.server.request.*
 import io.ktor.server.response.*
@@ -26,8 +25,8 @@ fun Route.customerRoutes() {
 
                 val individual = CreateCustomerRequest.Individual.builder()
                     .customerType(IndividualCustomerFields.CustomerType.INDIVIDUAL)
-                    .platformCustomerId(json.requireText("platformCustomerId"))
                     .apply {
+                        json.optText("platformCustomerId")?.let { platformCustomerId(it) }
                         json.optText("fullName")?.let { fullName(it) }
                         json.optText("nationality")?.let { nationality(it) }
                         json.optText("birthDate")?.let { birthDate(LocalDate.parse(it)) }