Fix failing integration test for currency missing-symbol behavior

[wilsonfreitas]

Context

The live integration suite currently fails because tests/integration/test_currency.py::test_currency_get_symbol still expects _get_symbol("ZAR") to return None, while the current implementation raises CurrencyNotFoundError.

Scope

Update the integration test to match the current missing-symbol contract. Keep the test focused on public behavior where possible.

Branching

Create the implementation branch from project-health-hardening and merge the fix back into project-health-hardening before any merge to main.

Acceptance Criteria

• uv run pytest -m integration no longer fails for this deterministic stale expectation.
• The test still distinguishes real external API outages from behavior regressions.
• No product behavior is changed solely to satisfy the stale test.

[wilsonfreitas]

Merged via #45 into project-health-hardening. The stale currency integration expectation now matches the current CurrencyNotFoundError behavior.