fix(core): use backtick-escaped constant reference for enum default values

[mfabisiak]

When an enum value produces a constant name that is not a valid Kotlin
identifier (e.g. "1" → 1), the generated default value was emitted
without backticks, causing a compilation error:

data class Request(
    val floor: Floor = Floor.1  // invalid
)

Using %N instead of %L in KotlinPoet's CodeBlock lets the library
apply backtick escaping automatically:

data class Request(
    val floor: Floor = Floor.`1`  // correct
)

Adds a regression test covering a numeric-string enum default.

[Copilot]

Pull request overview

This PR fixes Kotlin model generation for enum-valued defaults when the generated enum constant name is not a valid Kotlin identifier (e.g., "1"), ensuring KotlinPoet emits a backtick-escaped constant reference so generated code compiles.

Changes:

• Switch enum default-value emission from KotlinPoet’s %L to %N so invalid identifiers get backtick-escaped automatically.
• Add a regression test that asserts numeric-string enum defaults produce backtick-escaped references.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
core/src/main/kotlin/com/avsystem/justworks/core/gen/model/ModelGenerator.kt	Uses %N for enum constant references in default values to trigger KotlinPoet escaping.
core/src/test/kotlin/com/avsystem/justworks/core/gen/ModelGeneratorTest.kt	Adds a regression test for a numeric-string enum default value to prevent reintroducing the compile error.

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[github-actions]

Coverage Report

Overall Project	96.53%	🍏
Files changed	100%	🍏

File	Coverage
ModelGenerator.kt	96.2%	🍏