Fix Swift oneOf discriminator decoding with enumUnknownDefaultCase

[lastMove]

Description

This PR implements discriminator-first decoding for oneOf schemas in Swift5 and Swift6 generators to fix issue #7549 where enumUnknownDefaultCase=true breaks discriminator-based routing.

Problem

When enumUnknownDefaultCase=true is set, discriminator fields in variant types have an unknownDefaultOpenApi fallback case. With the previous sequential try? decoding approach:

1. Decoder tries the first variant (e.g., PredicateBetween)
2. The discriminator field type doesn't match (e.g., "matchesAny" ≠ "between")
3. BUT the unknownDefaultOpenApi fallback accepts it - decode succeeds!
4. Wrong variant is selected with corrupted data
5. Correct variant (e.g., PredicateMatchesAny) is never tried

Solution

Implement discriminator-first decoding:

1. When discriminator exists: Read discriminator value FIRST using a keyed container, then switch on it to route directly to the correct variant
2. When no discriminator: Use the original sequential try? approach (backward compatible)

Changes

Templates Modified:

• modules/openapi-generator/src/main/resources/swift5/modelOneOf.mustache
• modules/openapi-generator/src/main/resources/swift6/modelOneOf.mustache

Key Implementation Details:

• Added DiscriminatorCodingKey enum for reading discriminator field
• Conditional logic: discriminator-first vs sequential try?
• Improved error messages that include actual discriminator value
• Fixed encoding bug: removed unused parameter from unknownDefaultOpenApi case
• Updated samples to reflect template changes

Example Generated Code (with discriminator):

public init(from decoder: Decoder) throws {
    // Discriminator-based decoding: read discriminator value first
    let keyedContainer = try decoder.container(keyedBy: DiscriminatorCodingKey.self)
    let discriminatorValue = try keyedContainer.decode(String.self, forKey: .type)

    switch discriminatorValue {
    case "between":
        self = .typePredicateBetween(try PredicateBetween(from: decoder))
    case "matchesAny":
        self = .typePredicateMatchesAny(try PredicateMatchesAny(from: decoder))
    default:
        throw DecodingError.dataCorrupted(...)
    }
}

Benefits

• ✅ Fixes discriminator-based oneOf decoding when enumUnknownDefaultCase=true
• ✅ Better performance: O(1) switch vs O(n) sequential tries
• ✅ Clearer error messages with actual discriminator values
• ✅ No breaking changes - backward compatible for non-discriminator schemas
• ✅ Follows Swift best practices for Codable

PR Checklist

• Read the contribution guidelines
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work
• Regenerated samples:

  ./mvnw clean install -DskipTests -Dmaven.javadoc.skip=true
  ./bin/generate-samples.sh bin/configs/swift5-oneOf.yaml
  ./bin/generate-samples.sh bin/configs/swift6-oneOf.yaml
  

  Committed all changed files including updated samples
• Filed PR against the correct branch: master (non-breaking change)
• PR solves a reported issue: fixes [BUG] [Swift5] support for oneOf, anyOf, allOf, and discriminator directives #7549
• Mentioning technical committee: @4brunu

cc @4brunu - This PR implements discriminator support for Swift oneOf schemas as discussed in #7549. The implementation uses discriminator-first decoding to avoid the bug where unknownDefaultOpenApi fallback cases break discriminator routing. Backward compatibility is maintained for non-discriminator oneOf schemas.

---

Summary by cubic

Fix Swift oneOf decoding when enumUnknownDefaultCase=true by reading the discriminator first and routing to the correct variant. Keeps non-discriminator schemas using the original sequential decoding; fixes #7549.

• Bug Fixes
  • Decode the discriminator via a keyed container and switch directly to the mapped variant.
  • Use sequential try? only when no discriminator is present.
  • For unknown discriminator values, return unknownDefaultOpenApi (if enabled) or throw with clearer error messages.
  • Remove the unused payload from unknownDefaultOpenApi during encoding.
  • Update Swift5/Swift6 modelOneOf templates and refresh samples.

^{Written for commit 96b6a88. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 4 files

[4brunu]

Hi @lastMove, thanks for the contribution.
Did you tested those changes locally?

[lastMove]

@4brunu yes I tested it, it works!
I think it would be better if I add a new test case for this. I will do that.

[4brunu]

That would be nice, thanks

[4brunu]

@lastMove can you merge master to your branch please? Thanks