Allow SetAccountCurrency after SetCash without throwing

[AlexCatarino]

Description

Changes SecurityPortfolioManager.SetAccountCurrency so it no longer throws when called after SetCash. The base account currency is switched in place:

• Different currency: the previous Cash entry stays in the CashBook with its existing balance, the new account currency starts at zero (or startingCash if provided), and a trace is logged noting the residual balance held in the previous currency. This undoes the automatic amount migration performed by CashBook.AccountCurrency's setter so balances stay in their original currency.
• Same currency: an optional startingCash overrides the previously set amount and a trace is logged stating the new amount and the prior amount. When startingCash is omitted or equals the existing amount, nothing is logged.
• No prior SetCash: behaves as before — only the existing "setting account currency to X" trace is emitted.

Related Issue

N/A

Motivation and Context

The previous behavior forced users to order SetAccountCurrency strictly before SetCash, throwing InvalidOperationException otherwise. Switching to a graceful in-place change with a clear log message is more forgiving and lets users override the starting amount when re-asserting the same account currency.

Requires Documentation Change

No.

How Has This Been Tested?

Replaced the obsolete CanNotChangeAccountCurrencyAfterSettingCash test (which expected the exception) with four new tests in SecurityPortfolioManagerTests:

• ChangeAccountCurrencyAfterSettingCashKeepsPreviousCash (covers both SetCash overloads) — verifies the previous balance is retained in the CashBook after a currency change.
• ChangeAccountCurrencyAfterSettingCashAppliesStartingCashToNewCurrency — verifies startingCash applies to the new currency while the previous balance is preserved.
• SetAccountCurrencyWithSameCurrencyOverridesStartingCash — verifies a same-currency call with startingCash overwrites the prior amount.
• SetAccountCurrencyWithSameCurrencyAndNoStartingCashKeepsExistingAmount — verifies a no-op same-currency call leaves the existing amount untouched.

All 8 SetAccountCurrency-related tests pass locally.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Refactor (non-breaking change which improves implementation)
• Performance (non-breaking change which improves performance. Please add associated performance test and results)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Non-functional change (xml comments/documentation/etc)

Checklist:

• My code follows the code style of this project.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• My branch follows the naming convention bug-<issue#>-<description> or feature-<issue#>-<description>