Make requests Encodable-only instead of Codable

[Copilot]

This PR changes WPNRequestBase and the generic parameter T in WPNRequest<T> from Codable to Encodable only, reducing constraints on integrators.

Changes Made

• Changed WPNRequestBase: Codable to WPNRequestBase: Encodable
• Changed WPNRequest<T: Codable> to WPNRequest<T: Encodable>
• Removed the unused init(from decoder:) method from WPNRequest class

Benefits for Integrators

Before (required Codable):

struct MyRequestPayload: Codable {  // Had to be Codable
    let userId: String
    let amount: Double
    
    // Had to implement init(from decoder:) even though it's never used!
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        userId = try container.decode(String.self, forKey: .userId)
        amount = try container.decode(Double.self, forKey: .amount)
    }
    // ... plus encode(to:) and CodingKeys
}

After (only requires Encodable):

struct MyRequestPayload: Encodable {  // Only needs to be Encodable!
    let userId: String
    let amount: Double
    
    // No need for init(from decoder:) - much cleaner!
    // Automatic synthesis of encode(to:) works perfectly
}

Rationale

Request objects are only encoded (sent to server), never decoded (received from server). Requiring Codable forced integrators to implement unnecessary Decodable conformance. This change:

• ✅ Reduces boilerplate code for integrators
• ✅ Makes the API intent clearer (requests are only sent, not received)
• ✅ Maintains full backward compatibility
• ✅ Keeps response types as Codable (which need both encoding and decoding)

Fixes #71.

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[kober32]

Note the change into the 1.5.1 version in changelog.md file

[Copilot]

Note the change into the 1.5.1 version in changelog.md file

Added changelog.md with the 1.5.1 entry documenting the Encodable-only changes. Commit: 019f29a

[kober32]

OK