feat: update Rates to use readonly properties

[owenvoke]

This updates to use readonly public properties for Rates, and improves type hinting further.

[Copilot]

Pull request overview

This PR modernizes the package’s value objects and providers by adopting PHP’s readonly features and improving PHPStan type hints, while keeping backwards compatibility via deprecated accessors on Rates.

Changes:

• Convert Worksome\Exchange\Support\Rates into a final readonly DTO with public readonly properties and deprecate the old getters.
• Mark multiple exchange rate provider classes (and ValidateCurrencyCodes) as final readonly to better express immutability.
• Improve phpdoc types from non-empty-array<int, string> to non-empty-list<string> and update command/docs to use property access.

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/Support/Rates.php	Makes Rates a readonly DTO with public properties and adds deprecations for legacy getters.
src/Support/FlatCurrencyCodeProvider.php	Adds {@inheritdoc} docblock for all().
src/ExchangeServiceProvider.php	Minor instantiation syntax simplification in the binding closure.
src/ExchangeRateProviders/NullProvider.php	Marks provider as final readonly.
src/ExchangeRateProviders/FrankfurterProvider.php	Marks provider as final readonly.
src/ExchangeRateProviders/FixerProvider.php	Marks provider as final readonly.
src/ExchangeRateProviders/ExchangeRateHostProvider.php	Marks provider as final readonly.
src/ExchangeRateProviders/CurrencyGEOProvider.php	Marks provider as final readonly.
src/ExchangeRateProviders/CachedProvider.php	Marks provider as final readonly.
src/Exchange.php	Makes validator dependency readonly; updates doc types; minor instantiation syntax simplification.
src/Contracts/ExchangeRateProvider.php	Updates phpdoc to non-empty-list<string>.
src/Contracts/CurrencyCodeProvider.php	Updates phpdoc to non-empty-list<string>.
src/Contracts/Actions/ValidatesCurrencyCodes.php	Updates phpdoc param/return types to non-empty-list<string>.
src/Commands/ViewLatestRatesCommand.php	Tightens return shape typing and switches rendering to use Rates public properties.
src/Commands/InstallCommand.php	Switches description string quoting.
src/Commands/Concerns/HasUsefulConsoleMethods.php	Collapses the mixin docblock formatting.
src/Actions/ValidateCurrencyCodes.php	Marks action as final readonly.
README.md	Updates usage example to read Rates via properties.

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