[Ktor] HttpException: fall back to default reason phrase

[rfc2822]

If there's no HTTP reason prhase (like for HTTP/2), this PR changes HttpException to fall back to a default reason phrase for the respective status code.

Previously, a HttpException for HTTP/2 403 had the message: HTTP 403.

Now it has HTTP 403 Forbidden as one would expect.

[Copilot]

Pull request overview

This PR enhances HttpException to include HTTP reason phrases in error messages, with a fallback mechanism for protocols like HTTP/2 that don't provide them. The change improves error message clarity by transforming messages from "HTTP 403" to "HTTP 403 Forbidden".

• Adds a reasonPhrase() helper method that extracts the status description or falls back to the default phrase for standard status codes
• Updates the exception message format to include both the status code value and its description
• Adds comprehensive test coverage for the new functionality

Reviewed changes

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

File	Description
src/main/kotlin/at/bitfire/dav4jvm/ktor/exception/HttpException.kt	Implements the reasonPhrase() method and updates the message construction to use it
src/test/kotlin/at/bitfire/dav4jvm/ktor/exception/HttpExceptionTest.kt	Adds tests for the message format and reasonPhrase() method with both empty and custom descriptions

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[rfc2822]

Works for bitfireAT/davx5-ose#1878, so merging