fix: add comprehensive tests for TTL extension idempotency

[NneOdogwu]

Issue

Repeated TTL extension should be idempotent and should not modify wrap contents.

Issue ID: fa64bb7d-3028-401d-8f82-777d0ca2a420

Changes

Added comprehensive test suite for extend_ttl function to verify idempotent behavior:

Tests Added

1. test_extend_ttl_idempotent - Core test verifying that calling extend_ttl multiple times does not modify any wrap data:

   • Mints a single wrap
   • Calls extend_ttl three times
   • Asserts that all wrap fields remain unchanged after each call
   • Verifies balance and latest period are preserved

2. test_extend_ttl_nonexistent_wrap_does_not_panic - Ensures graceful handling of nonexistent wraps:

   • Calls extend_ttl on a wrap that doesn't exist
   • Verifies no panic occurs
   • Confirms balance remains 0

3. test_extend_ttl_preserves_all_fields - Comprehensive field preservation check:

   • Verifies every field in WrapRecord is unchanged after TTL extension

4. test_extend_ttl_multiple_wraps - Tests idempotency across multiple wraps:

   • Mints two wraps with different periods
   • Extends both wraps multiple times
   • Verifies both wraps remain unchanged

Test Results

All 4 tests pass ✅

Acceptance Criteria ✅

• ✅ Mint one wrap and call extend_ttl multiple times
• ✅ Assert record data, balance, and latest period remain unchanged
• ✅ Verify TTL extension is idempotent

Key Findings

The extend_ttl function is already idempotent and does not modify wrap contents.
closes zintarh#257