Problem

CopilotClientOptions.Environment is a per-client dictionary. That model works for child-process transports (each client = its own OS process with its own environment) but is incoherent for the in-process (FFI) transport, where the native Rust runtime loads into the single shared .NET host process. A process has one environment block, so "per-client env vars" cannot be honored for host-resident native code — the option silently did nothing there.

What this PR does (C# / SDK slice)

SDK (dotnet/src)

• Add Environment to ChildProcessRuntimeConnection — per-client env where it's actually coherent (stdio/TCP). Clear+replace semantics, matching options.Environment for easy migration.
• Guards in CopilotClient (fail loud instead of fail silent):
  • throw if options.Environment is set with RuntimeConnection.ForInProcess()
  • throw if options.Telemetry is set with the in-process transport (telemetry lowers to env vars read host-side, so it can't be honored per-client in-process)
  • throw if both options.Environment and the connection's Environment are set

Test harness (dotnet/test)

• E2ETestContext.CreateClient gains an environment parameter that routes the environment to the connection (child-process) or mirrors it onto the host process (in-process), and rejects options.Environment.
• All E2E call sites converted off options.Environment onto the new parameter, dogfooding ChildProcessRuntimeConnection.Environment.
• Removed the useStdio shortcut (misleading false == TCP overload); the few transport-pinned tests now set options.Connection directly.
• Pinned the file-telemetry E2E test to stdio (telemetry is unsupported in-process under the new guard).

Scope / follow-ups

This is deliberately the SDK-side slice: it makes every previously-silent incoherence a loud error and gives child-process transports the correct API shape. It does not make in-process telemetry (or in-process env) actually work — that requires threading the exporter/env config through the FFI boundary as explicit per-connection config in the runtime, which is a separate change. Deprecating options.Environment outright is planned for v2.

Validation

• SDK + tests build clean; dotnet format --verify-no-changes passes.
• Representative converted tests pass under both stdio and in-process transports (incl. the CLI-path-dependent ConnectionTokenTests/SessionFsSqlite and the stdio-pinned telemetry test).