Add configurable PreserveServerUrlPath option for reverse proxy support

[Copilot]

When ServerUrl contains a base path (e.g. https://proxy.example.com/deepl/), the path component is preserved during URI resolution since relative API paths don't have leading slashes. This PR makes that behavior explicitly configurable via PreserveServerUrlPath so users can opt out if needed.

Changes

• TranslatorOptions: New PreserveServerUrlPath property (default true), inherited by DeepLClientOptions
• DeepLHttpClient: Centralized URI construction into ResolveUri() — when false, prepends / to force RFC 3986 absolute-path resolution, discarding the base path
• Tests: Coverage for both true/false behavior via TranslatorOptions and DeepLClientOptions

Usage

// Reverse proxy with path prefix — works by default (PreserveServerUrlPath = true)
var options = new DeepLClientOptions {
    ServerUrl = "https://proxy.example.com/deepl/"
};
// Requests go to https://proxy.example.com/deepl/v2/translate

// Opt out to resolve from server root
var options = new DeepLClientOptions {
    ServerUrl = "https://proxy.example.com/deepl/",
    PreserveServerUrlPath = false
};
// Requests go to https://proxy.example.com/v2/translate

[DeeJayTC]

actually....not sure wether the current behaviour is true or false...

[vernou]

The commit b07280e fixes the issue #73. This PR adds a parameter to retrieve the old behavior.

---

I don't think it's necessary to make this behavior configurable. The old behavior is misleading, so new user will not understand the problem.

If a developper want to remove the server url path, then he can remove it before to set ServerUrlPath.