Fix: Add explicit swift-service-context dependency for Xcode 26 transitive dylib bug

[stovak]

Problem

Xcode 26 introduced a regression in Swift Package Manager builds where packages built as dynamic frameworks don't properly propagate transitive framework dependencies during the link phase.

Symptoms

When AsyncHTTPClient is used as a transitive dependency (i.e., Package A → Package B → AsyncHTTPClient), the build fails with:

Undefined symbols for architecture arm64:
  "_$s20ServiceContextModule0aB3KeyPTl", referenced from:
      _$s17AsyncHTTPClient0bC0V7executeyyAA15HTTPExecutorJobCYaKF in AsyncHTTPClient.o

Root Cause

• Xcode 26 builds all "automatic" SPM products as dynamic frameworks (*.framework)
• AsyncHTTPClient uses symbols from ServiceContextModule (via inlined code from Tracing)
• However, AsyncHTTPClient's Package.swift doesn't declare swift-service-context as a direct dependency
• It only declares swift-distributed-tracing, which itself depends on swift-service-context
• Xcode 26 correctly links ServiceContextModule.framework into Tracing.framework
• BUT Xcode 26 fails to propagate this transitive dependency when linking AsyncHTTPClient.framework
• Result: AsyncHTTPClient.o references ServiceContextModule symbols but the framework doesn't link against it

Solution

Add explicit package-level and target-level dependencies for swift-service-context to ensure Xcode 26 can link the framework correctly.

Changes

• Added .package(url: "https://github.com/apple/swift-service-context.git", from: "1.1.0") to package dependencies
• Added .product(name: "ServiceContextModule", package: "swift-service-context") to AsyncHTTPClient target dependencies
• Added same to AsyncHTTPClientTests target for test builds

Impact

• No functional changes - ServiceContextModule was already available transitively through Tracing
• Fixes builds for projects using AsyncHTTPClient as a transitive dependency under Xcode 26
• No impact on Xcode 25 or earlier (explicit dependency is redundant but harmless)
• No impact on Linux/Swift CLI builds (they don't have the transitive dylib bug)

Testing

Verified that this fixes builds for projects with dependency chains like:

App → SwiftBruja → VoxAlta → AsyncHTTPClient

Previously failed with undefined symbols, now builds and links correctly.

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Related Issue: This is a workaround for an Xcode 26 regression. Ideally Apple would fix the SPM dylib linking behavior, but adding explicit dependencies is a reasonable defensive practice regardless.