Add configurable missing localization behavior

[Copilot]

Implements configurable behavior when a localization key is not found (issue #38). Users can now choose to ignore (default), log a warning, or throw an exception.

Changes

• MissingLocalizationBehavior enum - Defines Ignore, LogWarning, ThrowException options
• MissingLocalizationException - Custom exception with key, culture, and searched location details
• JsonLocalizationOptions.MissingLocalizationBehavior - New property (defaults to Ignore for backward compatibility)
• JsonStringLocalizer - Handles missing localization based on configured behavior
• JsonStringLocalizerLoggerExtensions - Added warning log message for missing localizations

Usage

services.AddJsonLocalization(options =>
{
    options.ResourcesPath = "Resources";
    options.MissingLocalizationBehavior = MissingLocalizationBehavior.ThrowException;
});

When a key is missing with ThrowException:

MissingLocalizationException: Localization for key 'MyKey' was not found for culture 'fr-FR' in '/path/to/resources'.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Missing Localization Behavior</issue_title>
<issue_description>We can:

• Add log entry

• Throw an exception

• Ignore and use the localization key</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Missing Localization Behavior #38

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