Refactor DictionaryEncoder/DictionaryDecoder to struct + Sendable #3

Summary

DictionaryEncoder and DictionaryDecoder are currently open class. They should be converted to public struct and made Sendable.

Proposed Changes

struct + Sendable

• open class DictionaryEncoder → public struct DictionaryEncoder: Sendable
• open class DictionaryDecoder → public struct DictionaryDecoder: Sendable
• open var properties → public var

userInfo

• userInfo: [CodingUserInfoKey: Any] → userInfo: [CodingUserInfoKey: any Sendable] on both types
• Update DictionaryEncoderOptions and DictionaryDecoderOptions to match
• Bridge [CodingUserInfoKey: any Sendable] → [CodingUserInfoKey: Any] in DictionaryEncoderImpl/DictionaryDecoderImpl to satisfy the Encoder/Decoder protocol requirement

Remove DateEncodingStrategy / DateDecodingStrategy

Unlike JSON, [String: Any] can hold a Date value natively — no serialization to string or number is needed. The strategies .secondsSince1970, .millisecondsSince1970, .iso8601, and .formatted(DateFormatter) only make sense for formats that cannot represent Date directly. For a dictionary, .deferredToDate is the only meaningful strategy, so the entire DateEncodingStrategy/DateDecodingStrategy enums can be removed. This also eliminates the DateFormatter / Sendable problem entirely and allows the struct to conform to Sendable without @unchecked.

Other Sendable fixes

• MissingValueDecodingStrategy.useDefault(defaults: [String: Any]) → [String: any Sendable]
• Add @Sendable to all closure-typed enum cases (DataEncodingStrategy.custom, KeyEncodingStrategy.custom, DataDecodingStrategy.custom, KeyDecodingStrategy.custom)

Files to Update

• Sources/DictionaryCoding/DictionaryEncoder.swift
• Sources/DictionaryCoding/DictionaryDecoder.swift
• Sources/DictionaryCoding/DictionaryEncoderOptions.swift
• Sources/DictionaryCoding/DictionaryDecoderOptions.swift
• Sources/DictionaryCoding/DictionaryEncoderImpl.swift
• Sources/DictionaryCoding/DictionaryDecoderImpl.swift
• Sources/DictionaryCoding/DictionaryEncoderImpl+Box.swift
• Sources/DictionaryCoding/DictionaryDecoderImpl+UnboxDecodable.swift
• Sources/DictionaryCoding/ISO8601DateFormatter+Shared.swift (can be deleted)