C#: Fix some Ql4Ql violations.

[michaelnebel]

Fix some Ql4Ql violations based on the following checks

• ql/field-only-used-in-charpred
• ql/could-be-cast
• ql/counting-to-zero
• ql/dataflow-module-naming-convention
• ql/if-with-none
• ql/missing-parameter-qldoc
• ql/misspelling

DCA looks good.

[Copilot]

Pull Request Overview

This PR fixes various Ql4Ql violations identified by automated checks, focusing on code quality improvements in the C# CodeQL library. The changes include refactoring to use more idiomatic CodeQL patterns, fixing spelling errors, and improving code documentation.

Key changes:

• Replace exists patterns with more concise instanceof checks
• Fix spelling errors in class names and predicates
• Move field declarations inside exists clauses to improve scoping
• Add missing parameter documentation

Reviewed Changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
DatabaseQuality.qll	Replace exists with instanceof for cleaner type checking
UselessUpcast.ql	Move field declaration inside exists clause for better scoping
MagicConstants.qll	Fix spelling error from "occurence" to "occurrence"
Deserializers.qll	Fix spelling error in class name and add deprecated alias
UnsafeDeserializationQuery.qll	Update reference to corrected class name
SsaImpl.qll	Add missing parameter documentation
SSA.qll	Remove unused field by using wildcard in exists clause
Completion.qll	Replace field with inline any() expression
PrintAst.qll	Refactor to use any() pattern instead of field
Member.qll	Add missing parameter documentation
Callable.qll	Fix spelling error "substraction" to "subtraction"
Assignable.qll	Add missing parameter documentation
printAst.ql	Fix documentation grammar

Comments suppressed due to low confidence (1)

csharp/ql/lib/printAst.ql:22

• Grammar error: 'the element is e is fromSource' should be 'the element e is fromSource' (remove duplicate 'is').

   * Holds if the location `l` matches the selected file in the VS Code extension and
   * the element is `e` is `fromSource`.