fix(generator): treat string const as single-value enum (closes #10)

[lightsofapollo]

Summary

• Treat { "type": "string", "const": "X" } (no enum array) as a degenerate single-value enum so the generator emits a tightly-typed single-variant enum instead of Option<String>.
• Centralized in SchemaDetails::is_string_enum / string_enum_values so all four call sites (typed-string, untyped-string, property-with-context, inferred-string) pick up the change.

Before / After

For the spec from #10:

"ConstModifier": {
  "type": "object",
  "properties": {
    "someConstant": { "type": "string", "const": "TheOnlyValidValue" }
  }
}

Before:

pub struct ConstModifier {
    #[serde(rename = "someConstant", skip_serializing_if = "Option::is_none")]
    pub some_constant: Option<String>,
}

After:

pub struct ConstModifier {
    #[serde(rename = "someConstant", skip_serializing_if = "Option::is_none")]
    pub some_constant: Option<ConstModifierSomeConstant>,
}

#[derive(Debug, Clone, PartialEq, Eq, Deserialize, Serialize, Default)]
pub enum ConstModifierSomeConstant {
    #[default]
    #[serde(rename = "TheOnlyValidValue")]
    TheOnlyValidValue,
}

Construction:

let m = ConstModifier {
    some_constant: Some(ConstModifierSomeConstant::TheOnlyValidValue),
};
// or simply: ConstModifier::default()

Discriminator paths are unaffected

• extract_inline_discriminator_value (src/analysis.rs:2507) reads const_value directly when picking variant tags.
• The variant struct's discriminator field is filtered out of the generated struct in src/generator.rs:932-947, so this change does not affect (de)serialization of tagged unions.

Snapshot drift

Three pre-existing snapshots are updated, all in the desired direction (bare String → single-variant enum on standalone structs):

• debug_test.snap
• type_property_only_test.snap
• oneof_in_property.snap (ImageBlock.type field — ImageBlock itself is not a discriminated-union variant)
• array_union_items.snap

The oneof_in_property and array_union_items snapshots also pick up some orphan single-variant enums (e.g., URLImageSourceType, Base64ImageSourceType) for discriminator fields whose enums are no longer referenced by their parent struct. This is a pre-existing behavior — visible today in discriminator_no_mapping.snap (DogSpecies / CatSpecies) for any enum: ["X"] discriminator field. Tracked separately in #11; intentionally out of scope here.

Test plan

• New test tests/const_only_property_test.rs reproducing Overly Broad Type Generated When Using "const" #10 (both required and optional cases) — both pass.
• Full cargo test suite — all green.
• cargo fmt — clean.
• Reviewed all snapshot diffs — only intended changes, no regressions.

🤖 Generated with Claude Code