Add HttpClient.ValidateAfterInactivityMs to custom runtime settings

[mxsriaz]

This MR adds the HttpClient.ValidateAfterInactivityMs custom runtime setting to the custom settings reference.

• Key: HttpClient.ValidateAfterInactivityMs
• Default: disabled when unset; Apache HttpClient default behavior applies
• Applies to: Call REST Service and Call Web Service activities
• Introduced in: Mendix 10.24, 11.6, and 11.12

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