Harden JSONEncoder.wrapEncodable + dictionary encoding regression test (skip-foundation#62)

[vincentborko]

Companion to the root fix in skiptools/skip-lib#46. This PR is the skip-foundation half of #62.

Motivation

Encoding a Codable type with a populated dictionary property can crash with:

skip.lib.ErrorException: java.lang.ClassCastException: skip.lib.Tuple2 cannot be cast to skip.lib.Encodable
    at skip.foundation._SpecialTreatmentEncoder.wrapEncodable(JSONEncoder.kt)

The root cause is in skip-lib's type-erased Codable dispatch (a Dictionary is also a Sequence<Tuple2>, so it was iterated as raw pairs) and is fixed in skiptools/skip-lib#46. This PR adds two things on the skip-foundation side.

1. Defense-in-depth in wrapEncodable

On SKIP the generic bound of wrapEncodable<E: Encodable> is weakened to E: Any (see the // SKIP DECLARE above it), so (encodable as Encodable).encode(...) is an unchecked downcast: any future value reaching this branch that is not Encodable fails with a bare ClassCastException naming nothing. Cast defensively and throw an EncodingError naming the concrete offending type instead — mirroring the EncodingError.invalidValue already thrown on the top-level encode path, and the "fail loud with an actionable message" pattern used elsewhere in the runtime. Behaviour is unchanged for genuine Encodable values.

2. Regression test

Adds a JSONEncoder round-trip/encode test (testDictionaryCodableTypeErased) covering an optional [String: String]? (the literal #62 shape, reached through encodeIfPresent) and nested [String: [String: String]] dictionaries — shapes not previously exercised (the suite only had non-optional dictionaries, which already worked). Against unpatched skip-lib this reproduces the Tuple2 cannot be cast to Encodable crash on Robolectric; with skip-lib#46 it passes and produces the correct JSON object shape.

The two nested fixtures are Encodable-only, to isolate the #62 encode crash from an unrelated nested-dictionary decode codegen limitation (the transpiled decoder types the inner dictionary as Dictionary<*, *>).

Sequencing / CI note

The new test only passes once skip-lib#46 is released and this package's skip-lib dependency is bumped to include it. Until then it will fail against the currently pinned skip-lib 1.4.0 (reproducing the crash). The wrapEncodable hardening in part 1 is independent and safe to land on its own. Sequencing is yours to decide — happy to split the test out into a follow-up if you'd prefer to merge the hardening first.

Testing

• swift test (native + transpiled/Robolectric), building against skip-lib#46 as a local dependency: the full TestJSON suite passes (5/5), including the new case; against unpatched skip-lib the new case reproduces the crash.

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• I have read and agree to the Contributor License Agreement.
• swift test runs locally.
• This change was prepared with AI assistance (Claude) and reviewed by me.