C#: Add support for MaD barriers and barrier guards.

[aschackmull]

What the title says. This adds the basic plumbing, but doesn't update any queries yet.

[Copilot]

Pull request overview

This PR adds support for MaD (Models-as-Data) barriers and barrier guards in C#. This is foundational infrastructure that enables modeling of security barriers and barrier guards through external data models, though queries are not updated yet to use this functionality.

Changes:

• Added ParameterizedBarrierGuard module for parameterized barrier guard support
• Implemented barrierElement and barrierGuardModel predicates to interpret MaD barrier specifications
• Extended model validation to cover barrier and barrier guard models
• Added public API for barrier nodes

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
csharp/ql/lib/semmle/code/csharp/dataflow/internal/SsaImpl.qll	Adds ParameterizedBarrierGuard module with signature support for parameterized guard checks
csharp/ql/lib/semmle/code/csharp/dataflow/internal/FlowSummaryImpl.qll	Implements barrierElement and barrierGuardElement predicates to interpret barrier models from MaD specifications
csharp/ql/lib/semmle/code/csharp/dataflow/internal/ExternalFlow.qll	Adds validation logic for barrier models, accepting value conversion, and cached barrier node predicates
csharp/ql/lib/semmle/code/csharp/dataflow/internal/DataFlowPublic.qll	Exposes public API for ParameterizedBarrierGuard module with comprehensive documentation

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