C#: Improve cs/dereference-* queries and add to the Code Quality suite.

[michaelnebel]

In this PR we do the following for cs/dereferenced-value-is-always-null and cs/dereferenced-value-may-be-null.

• Change the class of de-referenced expressions: When calling a library extension method where the this parameter is a nullable type, we now assume that the null may happen in the extension method itself (this is at least common for the usage of testing frameworks, which reports a high amount of false positives related to the use of such extension methods).
• Remove the explicit type requirements on Debug.Assert methods (they are trivially true). They cause issues when extraction have broken types - and the change doesn't affect the methods that match the predicate when types are healthy.
• Add the queries of the Code Quality suite.
• Convert the tests to use inline expectations tests.

DCA looks good.

[Copilot]

Pull Request Overview

This PR enhances the null-dereference queries by treating nullable extension-method receivers as potentially safe, removes strict type checks on Debug.Assert, and folds the new queries into the Code Quality suite with inline-expectation tests.

• Broaden cs/dereferenced-value-is-always-null/cs/dereferenced-value-may-be-null to ignore null dereferences inside library extension methods on nullable receivers.
• Simplify Debug.Assert matching by dropping parameter-type checks.
• Add both queries to the Code Quality query suite and convert tests to use inline expectations.

Reviewed Changes

Copilot reviewed 26 out of 26 changed files in this pull request and generated no comments.

File	Description
csharp/ql/src/change-notes/2025-06-03-dereferece-extension-method.md	Adds change note for extension-method deref behavior (typo in name)
csharp/ql/lib/semmle/code/csharp/frameworks/system/Diagnostics.qll	Loosen Assert matcher to any Assert overload without param check
csharp/ql/lib/semmle/code/csharp/dataflow/Nullness.qll	Update Dereference predicate to treat nullable extension receivers safely
csharp/ql/integration-tests/posix/query-suite/csharp-code-quality.qls.expected	Include the new queries in the Code Quality suite

[hvitved]

LGTM 🎉