fix(ios): Guard against nil NSData and pin tests to SPI import

Two fixes flagged by Warden:

1. `RNSentryInternal.envelope(fromData:)` accepted a non-optional Swift
   `Data`. If the ObjC caller passed a nil `NSData*` (e.g. from a failed
   base64 decode of the envelope payload), the ObjC→Swift bridge would
   force-unwrap it and crash before the method body ran. Change the
   parameter to `Data?` with an internal `guard let` — matches the
   nil-tolerant behaviour of the deprecated
   `PrivateSentrySDKOnly.envelopeWithData:`.

2. Swift Cocoa tests accessed `SentrySDK.internal.*` with plain
   `import Sentry`. The `.internal` accessor is public today, but the
   sub-APIs the tests touch (`options`, `appStart`, `performance`) sit
   next to `@_spi(Private)`-gated siblings on the same struct. Add
   `@_spi(Private) import Sentry` to `RNSentryStartTests.swift`,
   `RNSentryStartFromFileTests.swift`, and `RNSentryReplayOptionsTests.swift`
   so they keep compiling if the surface is ever tightened.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: alwx

packages/core/RNSentryCocoaTester/RNSentryCocoaTesterTests/RNSentryReplayOptionsTests.swift

@@ -1,4 +1,4 @@
-import Sentry
+@_spi(Private) import Sentry
 import XCTest
 
 // File length grows as replay option coverage is added; lint runs with `--strict`.

packages/core/RNSentryCocoaTester/RNSentryCocoaTesterTests/RNSentryStartFromFileTests.swift

@@ -1,4 +1,4 @@
-import Sentry
+@_spi(Private) import Sentry
 import XCTest
 
 final class RNSentryStartFromFileTests: XCTestCase {

packages/core/RNSentryCocoaTester/RNSentryCocoaTesterTests/RNSentryStartTests.swift

@@ -1,4 +1,4 @@
-import Sentry
+@_spi(Private) import Sentry
 import XCTest
 
 final class RNSentryStartTests: XCTestCase {

packages/core/ios/RNSentryInternal.swift

@@ -82,8 +82,13 @@ import Foundation
 
     // MARK: - Envelope
 
-    @_spi(Private) @objc public static func envelope(fromData data: Data) -> SentryEnvelope? {
-        SentrySDK.internal.envelope.deserialize(from: data)
+    // Accepts `Data?` (nil-safe) rather than `Data` so the ObjC bridge boundary
+    // doesn't force-unwrap a nil `NSData*` from a failed base64 decode — that
+    // would crash before we ever get a chance to check the result. Matches the
+    // nil-tolerant behaviour of the deprecated `PrivateSentrySDKOnly.envelopeWithData:`.
+    @_spi(Private) @objc public static func envelope(fromData data: Data?) -> SentryEnvelope? {
+        guard let data = data else { return nil }
+        return SentrySDK.internal.envelope.deserialize(from: data)
     }
 
     @_spi(Private) @objc public static func capture(_ envelope: SentryEnvelope) {