fix: correct typos in WebUI error message and XML doc comments

[Avicennasis]

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Summary

Fix 3 typos across 3 files. Each was verified against the current contents of master before changing.

• src/Rules.Framework.WebUI/Handlers/GetRulesHandler.cs (line 37): "Date begin cannot be greater than after" -> "Date begin cannot be greater than date end"
• src/Rules.Framework/Providers/InMemory/InMemoryProviderRulesDataSource.cs (line 9): a in-memory database -> an in-memory database
• src/Rules.Framework/Builder/IRuleBuilder.cs (line 48): <param name="conditionType">The content type.</param> -> <param name="conditionType">The condition type.</param>

Notes for reviewers

One change is to a user-facing string, not a comment - please review it deliberately. The GetRulesHandler message is returned in the HTTP 400 body when IsValidFilterDates fails. That guard (lines 71-76) returns false only when DateBegin > DateEnd, so "greater than after" appears to be an unfinished sentence; "date end" matches the naming used for the parameter elsewhere in the codebase (<param name="dateEnd">The date end.</param>). I grepped the whole repository, including tests/, for Date begin cannot be - this string occurs exactly once, so no test assertions needed updating. If you would rather word it differently, say so and I will amend.

The IRuleBuilder change corrects a copy-paste in the XML doc only. The documented parameter really is conditionType (see the WithCondition<TDataType> signature on line 52), and the neighbouring <param name="contentType">The content type.</param> on line 65 is correct as-is and untouched. No identifier was renamed.

Not included, in case you want it as a follow-up: the same a in-memory -> an in-memory slip also appears twice in src/Rules.Framework/Providers/InMemory/InMemoryProviderRulesDataSourceSelectorExtensions.cs (lines 13 and 25). I left those alone to keep this diff to the files listed above - happy to add them here if you would prefer a single sweep.

Commit message follows the conventional commits specification per CONTRIBUTING.md, and the branch targets master.