Add support for default values in Forms4s

[LSUDOKO]

implements support for default values in both Scala-derived forms and JSON Schema-derived forms. It ensures that default values defined in case classes or JSON schemas are correctly used to pre-fill forms on creation

forms4s-jsonschema:
Updated
FormFromJsonSchema.scala
to extract the default value from JSON Schema properties (specifically handling ExampleSingleValue) and pass it to the FormElement core.
Added a new test case in
FormFromJsonSchemaSpec.scala
to verify that default values from a JSON schema are correctly propagated to the generated form elements.

all tests passed
solve issue #8

Summary by CodeRabbit

• New Features

  • Added support for default values in form elements.
  • Automatically derives defaults from case class parameters.
  • Reads and applies defaults from JSON Schema definitions.
  • Form initialization now populates fields with their configured defaults.

• Tests

  • Added tests verifying default value propagation in generated form elements.
  • Added tests ensuring initial form state reflects specified defaults from models and JSON Schema.

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[coderabbitai]

📝 Walkthrough

Walkthrough

This PR adds compile-time default value extraction and propagation throughout the forms4s library. A new macro-based DefaultValues object extracts defaults from case classes, which are then threaded through form element creation and state initialization, replacing hardcoded defaults with element-specific values.

Changes

Cohort / File(s)	Summary
Macro infrastructure forms4s-core/src/main/scala/forms4s/DefaultValues.scala	New compile-time macro object that extracts default-valued fields from case classes using Scala 3 reflection, producing a Map[String, Any] of field names to their default value method references.
Form element structure forms4s-core/src/main/scala/forms4s/FormElement.scala	Added optional defaultValue: Option[Any] = None parameter to the Core case class to carry element-level default values.
Form state & derivation forms4s-core/src/main/scala/forms4s/FormElementState.scala, forms4s-core/src/main/scala/forms4s/ToFormElem.scala	Updated form state initialization to derive defaults from element cores instead of hardcoded literals (Text, Select, Checkbox, Number). Extended ToFormElem to invoke DefaultValues.extract, compute per-field defaults, and propagate them via updated updateCore signature.
JSON schema integration forms4s-jsonschema/src/main/scala/forms4s/jsonschema/FormFromJsonSchema.scala	Enhanced to extract default values from JSON schema and pass them into FormElement.Core constructor.
Test coverage forms4s-core/src/test/scala/forms4s/ToFormElemTest.scala, forms4s-jsonschema/src/test/scala/forms4s/jsonschema/FormFromJsonSchemaSpec.scala	Added tests validating that default values are correctly extracted from case class definitions and JSON schemas, and properly initialized in form elements and form state.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Poem

🐰 Defaults now flow through every form,
Extracted at compile time, fresh and warm,
From case class to state, each field knows its way,
No more hardcoded values to decay! 🎉

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and directly summarizes the main feature addition across the entire changeset.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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