Fix asset code case sensitivity - Stellar asset codes are case-sensitive but we were converting all codes to uppercase causing issues with trust lines

[Alonzo]

What

Fixed asset code case sensitivity in the CreateAsset handler. Previously, all asset codes were converted to uppercase using 'strings.ToUpper()', but Stellar asset codes are case-sensitive. This caused issues when users entered assets with lowercase letters leading to trustline mismatches. The fix preserves the exact case entered by users while still using case-insensitive comparison for native XLM asset validation.

Why

Stellar asset codes are case-sensitive identifiers. When users create assets with mixed case (like 'USDM0a'), the system was converting them to uppercase ('USDM0A'), which could cause issues when the actual asset on-chain has a different case or users try to add trustlines that don't match the stored case.

Known limitations

N/A

Checklist

• Title follows SDP-1234: Add new feature or Chore: Refactor package xyz format. The Jira ticket code was included if available.
• PR has a focused scope and doesn't mix features with refactoring
• Tests are included (if applicable)
• CHANGELOG.md is updated (if applicable)
• CONFIG/SECRETS changes are updated in helmcharts and deployments (if applicable)
• Preview deployment works as expected
• Ready for production

[philipliu]

Thanks for the fix. There are a few more places where asset codes were still case-insensitive. I've addressed it here #967