test: add integration test for ledgerToTimestamp (#461)

[Kaycee276]

Description

Closes #461

This PR adds an integration test for the ledgerToTimestamp helper to confirm its timestamp estimation accuracy using known, hardcoded reference values. This ensures the conversion logic is fully verified before the helper is adopted in the production UI.

The integration test explicitly verifies the helper against a known reference point (Ledger 50,000,000 at 2024-01-01T00:00:00.000Z) to satisfy all acceptance criteria:

• Equal Ledger: Asserts that passing a ledger equal to the reference returns the exact reference timestamp.
• Future Ledger: Asserts that passing a ledger 100 sequences ahead of the reference returns an estimated date exactly 500 seconds later.
• Past Ledger: Asserts that passing a ledger 100 sequences behind the reference returns an estimated date exactly 500 seconds earlier.

Changes

• Created a new test file: src/utils/__tests__/stellarLedger.integration.test.ts to cleanly separate the integration test with hardcoded data from the standard unit tests.

Verification

• Verified locally via pnpm test src/utils/__tests__/stellarLedger.integration.test.ts. All 3 tests pass successfully.

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