Implement Swift Concurrency for iOS Branch SDK (EMT-2309)

This commit introduces modern Swift Concurrency patterns to the iOS Branch SDK, mirroring the Android Kotlin Coroutines implementation. The changes enable async/await request processing, actor-based thread safety, and structured concurrency throughout the SDK.

## Swift Concurrency Implementation

Added three core Swift files using modern concurrency patterns:
- `BranchRequestOperation.swift`: Actor-isolated async operation for processing Branch server requests with structured concurrency, automatic main thread dispatching for callbacks, and session validation matching Android's implementation
- `BranchRequestQueue.swift`: Actor-based request queue with serial execution, async/await API, and thread-safe queue management replacing manual NSOperationQueue patterns
- `ConfigurationController.swift`: Swift configuration management with async initialization and proper error handling

Architecture parallels to Android:
- Android: CoroutineScope + Channel → iOS: Actor + AsyncStream
- Android: Dispatchers.IO → iOS: Task.detached
- Android: Dispatchers.Main → iOS: MainActor

## Swift Package Manager (SPM) Integration

Updated Package.swift to define dual-target architecture:
- `BranchObjCSDK`: Core Objective-C framework with all existing functionality
- `BranchSDK`: Swift wrapper exposing async/await APIs and importing BranchObjCSDK

This enables seamless Swift integration while maintaining full Objective-C compatibility.

## Objective-C Bridge Enhancements

Modified `BNCServerRequestOperation.m` to support Swift interop:
- Added Swift availability checks (@available decorators)
- Integrated async callback completion handling
- Maintained backward compatibility with existing Objective-C code

Updated `BNCServerRequestQueue.m` and `BNCRequestFactory.m` for improved Swift interaction.

## Public API Visibility Changes

Moved `BNCConfig.h` from Private to Public headers to enable SPM package visibility.

Added `BranchConstants.h` and `BranchConstants.m` to public BranchSDK directory for proper module exposure.

## Test Infrastructure Updates

Updated 16 test files to use modular imports:
- Changed from `#import "Branch.h"` to `@import BranchSDK`
- Updated bridging headers for Swift/Objective-C interop
- Modified test schemes to support SPM package dependencies

Test files updated:
- BNCAPIServerTest.m, BNCODMTests.m, BNCPasteboardTests.m
- BNCPreferenceHelperTests.m, BranchActivityItemTests.m
- BranchClassTests.m, BranchLoggerTests.m, BranchQRCodeTests.m
- BranchShareLinkTests.m, DispatchToIsolationQueueTests.m
- Branch_setBranchKeyTests.m
- UITestCaseMisc.m, UITestCaseTracking.m, UITestSendV2Event.m
- UITestSetIdentity.m, Branch-SDK-Tests-Bridging-Header.h

## Branch-TestBed Configuration

Updated Branch-TestBed Xcode project:
- Configured SPM package dependency reference
- Updated project.pbxproj with proper target dependencies
- Removed Package.resolved to allow flexible dependency resolution
- Updated all test schemes for SPM compatibility
- Modified AppDelegate.m and ViewControllers for async initialization

## Project Structure Cleanup

Organized source files into proper directory structure:
- Created `Sources/BranchSwiftSDK/` for Swift implementation
- Moved configuration files to appropriate locations
- Removed duplicate/temporary Swift files from previous iterations
- Updated both BranchSDK.xcodeproj and Package.swift configurations

## Key Features

- Structured concurrency with proper cancellation support
- Actor-based thread safety eliminating race conditions
- Automatic main thread dispatching for UI callbacks
- Session validation matching Android implementation
- Performance optimized with cooperative thread pool
- Serial request processing ensuring proper ordering
- Modern async/await API for Swift developers
- Full backward compatibility with existing Objective-C code

Author: wpinho-branch

Branch-TestBed/Branch-SDK-Tests/BNCAPIServerTest.m

@@ -11,7 +11,7 @@
 #import "BNCSystemObserver.h"
 #import "BNCConfig.h"
 #import "BranchConstants.h"
-#import "Branch.h"
+@import BranchSDK;
 
 @interface BNCAPIServerTest : XCTestCase
 

Branch-TestBed/Branch-SDK-Tests/BNCODMTests.m

@@ -7,7 +7,7 @@
 //
 
 #import <XCTest/XCTest.h>
-#import "Branch.h"
+@import BranchSDK;
 #import "BNCPreferenceHelper.h"
 #import "BNCRequestFactory.h"
 #import "BNCEncodingUtils.h"

Branch-TestBed/Branch-SDK-Tests/BNCPasteboardTests.m

@@ -8,7 +8,7 @@
 
 #import <XCTest/XCTest.h>
 #import "BNCPasteboard.h"
-#import "Branch.h"
+@import BranchSDK;
 
 @interface BNCPasteboardTests : XCTestCase
 

Branch-TestBed/Branch-SDK-Tests/BNCPreferenceHelperTests.m

@@ -9,7 +9,7 @@
 #import <XCTest/XCTest.h>
 #import "BNCPreferenceHelper.h"
 #import "BNCEncodingUtils.h"
-#import "Branch.h"
+@import BranchSDK;
 #import "BNCConfig.h"
 
 @interface BNCPreferenceHelper()

Branch-TestBed/Branch-SDK-Tests/Branch-SDK-Tests-Bridging-Header.h

@@ -2,4 +2,4 @@
 //  Module headers for Branch SDK unit testing.
 //
 
-#import "Branch.h"
+@import BranchSDK;

Branch-TestBed/Branch-SDK-Tests/BranchActivityItemTests.m

@@ -7,7 +7,7 @@
 //
 
 #import <XCTest/XCTest.h>
-#import "Branch.h"
+@import BranchSDK;
 
 @interface BranchActivityItemTests: XCTestCase
 @end

Branch-TestBed/Branch-SDK-Tests/BranchClassTests.m

@@ -7,7 +7,7 @@
 //
 
 #import <XCTest/XCTest.h>
-#import "Branch.h"
+@import BranchSDK;
 #import "BranchConstants.h"
 #import "BNCPasteboard.h"
 #import "BNCAppGroupsData.h"

Branch-TestBed/Branch-SDK-Tests/BranchLoggerTests.m

@@ -8,7 +8,7 @@
 
 #import <XCTest/XCTest.h>
 #import "BranchLogger.h"
-#import "Branch.h"
+@import BranchSDK;
 
 @interface BranchLoggerTests : XCTestCase
 

Branch-TestBed/Branch-SDK-Tests/BranchQRCodeTests.m

@@ -7,7 +7,7 @@
 //
 
 #import <XCTest/XCTest.h>
-#import "Branch.h"
+@import BranchSDK;
 #import "BranchQRCode.h"
 #import "BNCQRCodeCache.h"
 

Branch-TestBed/Branch-SDK-Tests/BranchShareLinkTests.m

@@ -9,7 +9,7 @@
 #import <XCTest/XCTest.h>
 #import "BranchShareLink.h"
 #import "BranchLinkProperties.h"
-#import "Branch.h"
+@import BranchSDK;
 
 @interface BranchShareLinkTests : XCTestCase