chore: sync Kotlin sample with paymentRail schema change (#280)

## Summary
- Updated Kotlin sample app to support the optional `paymentRail` field
added to quote account destinations in #279

## Context
The recent schema change (309c629) added an optional `paymentRail` field
to `AccountDestination` in quotes, allowing users to specify which
payment rail to use for transfers.

## Changes
**Kotlin Sample App:**
- Updated `buildQuoteDestination()` in `Quotes.kt` to pass through
`paymentRail` from input JSON when provided

**No changes needed:**
- Mintlify documentation: Examples demonstrate default behavior
(system-selected rail) which remains valid
- Grid Visualizer: Generated code uses default rail selection which
still works correctly

## Test plan
- [ ] Verify Kotlin sample accepts requests with optional `paymentRail`
field
- [ ] Verify existing requests without `paymentRail` continue to work

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: 3 people

samples/kotlin/src/main/kotlin/com/grid/sample/routes/Quotes.kt

@@ -122,6 +122,9 @@ private fun buildQuoteDestination(destNode: JsonNode): QuoteDestinationOneOf {
 
     val accountDest = QuoteDestinationOneOf.AccountDestination.builder()
         .accountId(destNode.get("accountId").asText())
+        .apply {
+            destNode.optText("paymentRail")?.let { paymentRail(it) }
+        }
         .build()
     return QuoteDestinationOneOf.ofAccountDestination(accountDest)
 }