Fix the cert4android user decision dialog not being shown

[sunkup]

Purpose

Fix the cert4android user decision dialog not being shown when needed.

Short description

Instead of passing a client engine directly to AppHttpClient we now pass a createEngine function which allows us to configure the engine at the same time. By default we create the minimally configured OkHttp engine and it gets replaced in the mock server which passes a function which creates a mock engine.

I tested (please double check), that:

• user agent is still showing
• redirects are handled by CalendarFetcher
• the timeout increase is applied

Note also that accepting a certificate from within the cert4android user decision dialog does not yet work. Probably due to the Nav3 rewrite. That is tracked in #697

Checklist

• The PR has a proper title, description and label.
• I have self-reviewed the PR.
• I have added documentation to complex functions and functions that can be used by other modules.
• I have added reasonable tests or consciously decided to not add tests.

[Copilot]

Pull Request Overview

This PR fixes certificate handling by changing the AppHttpClient architecture to properly initialize the certificate manager when the HTTP client engine is created.

• Replaced direct HttpClientEngine parameter with a factory function that creates engines with proper certificate configuration
• Updated MockServer to work with the new factory pattern while maintaining test isolation
• Simplified the engine configuration by moving SSL setup to the factory function

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
AppHttpClient.kt	Refactored to use engine factory function instead of direct engine, enabling proper certificate manager integration
ValidatorTest.kt	Updated test to use new factory function signature for mock HTTP client creation
MockServer.kt	Modified to provide engine factory function instead of direct engine instance for testing

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[ArnyminerZ]

Looks good, feel free to merge.