MLE-32022 Avoid null pointer exception when FailedRequest is null

[rjdew-progress]

Addresses GitHub issue #1969

[Copilot]

Pull request overview

Fixes a null-pointer exception when FailedRequest is absent by using an unknown server-status value.

Changes:

• Adds STATUS_UNKNOWN (-1).
• Handles missing failed-request status codes safely.
• Adds regression coverage for null FailedRequest handling.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
marklogic-client-api/src/test/java/com/marklogic/client/test/FailedRequestTest.java	Adds null-request regression test.
marklogic-client-api/src/main/java/com/marklogic/client/MarkLogicServerException.java	Prevents null status-code unboxing.
marklogic-client-api/src/main/java/com/marklogic/client/impl/RESTServices.java	Defines the unknown status constant.

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[rjrudin]

An interesting design choice as to what to return! I could see an argument for returning 500 - i.e. "Well we know the server complained about something, but we have no idea why for internal reasons, so here you go, have a 500" - but -1 seems reasonable as well as a way of communicating that we don't know what happened.