Expose apiKey on Purchases via @_spi(Internal) (#6635)

Adapter modules that import the SDK with `@_spi(Internal) import RevenueCat`
(e.g. `RevenueCatAdMob`) need to read the configured public API key when
constructing payloads such as the AdMob SSV `customRewardString`.

- Store the API key on `SystemInfo` and wire it through `Purchases` init.
- Add `@_spi(Internal) public var apiKey: String` on `Purchases`.
- Add a Swift API tester case so any future signature change is caught.
- Add a unit test verifying the accessor returns the configured key.

The key is already public plaintext in the app binary, so exposing it as SPI
does not introduce any new security risk.

Sources/Networking/HTTPClient/HTTPClient.swift

@@ -23,7 +23,6 @@ class HTTPClient {
 
     let systemInfo: SystemInfo
     let timeout: TimeInterval
-    let apiKey: String
     let authHeaders: RequestHeaders
 
     private let session: URLSession
@@ -43,8 +42,7 @@ class HTTPClient {
         TimeInterval(3)
     ]
 
-    init(apiKey: String,
-         systemInfo: SystemInfo,
+    init(systemInfo: SystemInfo,
          eTagManager: ETagManager,
          signing: SigningType,
          diagnosticsTracker: DiagnosticsTrackerType?,
@@ -70,8 +68,7 @@ class HTTPClient {
         self.dnsChecker = dnsChecker
         self.retriableStatusCodes = retriableStatusCodes
         self.timeout = requestTimeout
-        self.apiKey = apiKey
-        self.authHeaders = HTTPClient.authorizationHeader(withAPIKey: apiKey)
+        self.authHeaders = HTTPClient.authorizationHeader(withAPIKey: systemInfo.apiKey)
         self.dateProvider = dateProvider
         self.operationDispatcher = operationDispatcher
         self.requestTimeoutManager = timeoutManager ?? HTTPRequestTimeoutManager(