[rchatham]

Summary

• Import FoundationNetworking conditionally for JSON.fetch's URLSession, URLRequest and HTTPURLResponse types on Linux; Apple behavior is unchanged.
• Add five network-free fetch tests covering URL and custom POST request overloads, parsed HTTP error bodies, invalid JSON and transport error propagation. Each test owns an ephemeral session; the mock has no shared mutable state.
• Linux Swift 5.10 also lacks the async URLSession.data(for:) API. Provide a cancellation-aware callback bridge only for Linux compilers older than Swift 6; Apple and Swift 6 use the unchanged native API. Add a preflight cancellation test scoped to that shim.

Verification

• Reproduced the unmodified main revision's Linux core build failure in swift:6.0-jammy on the existing dev server: URLSession unavailable/missing networking types; exit 1.
• Final full suites: 451 tests, zero failures on Linux Swift 5.10; 450 tests, zero failures on Linux Swift 6.0.3 and macOS (baseline 445). Linux core build also passes.
• The first PR iteration's Swift 5.10 CI failure exposed the missing async API and led to the compatibility bridge. A cancellation assertion initially applied to all platforms was narrowed to the shim: native Swift 6 Linux may complete an immediate mock response before cancellation. No stronger native cancellation contract or production behavior change is claimed.
• Code review completed. No live API requests, credentials, releases or deployments.

Consumers

Unblocks LangTools PR rchatham/LangTools.swift#71. Consumers will use an immutable commit pin until a separately approved JSON release is available. No automatic merges or releases.

Final upstream CI matrix passed all four macOS/Linux Swift 5.10/6.0 jobs.