♻️ Convert Model generator to Stencil template

[MadsBogeskov]

Problem

Model.toSwift() uses inline string interpolation for the file wrapper.

Solution

Extract file wrapper to Model.stencil, keep complex body logic in Swift.

Testing

All 70 tests pass including golden file tests.

Stack

#169 → #179 → #180 → #181 → this PR → …

IOS-432

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Note

Low Risk
Low risk refactor that changes how model Swift source is wrapped (imports/extension/#if DEBUG) but not the generated model body logic; main risk is subtle whitespace/format differences affecting generated output or golden tests.

Overview
Refactors Model.toSwift() to stop building the outer Swift file via manual string concatenation and instead render a new Model.stencil template.

The template now controls imports, optional extension {{ serviceName }} wrapping/indentation, and #if DEBUG gating for isInternalOnly, while Swift continues to generate the core modelBody and passes rendering context (embedded, packagesToImport, etc.) into the template renderer.

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