fix(dotnet): merge env vars instead of replacing inherited environment

[PureWeen]

Summary

Fixes the bug reported in PureWeen/PolyPilot#441 (the PureWeen/PolyPilot issue tracker, not github/copilot-sdk).

In the .NET SDK, CopilotClientOptions.Environment silently clears and replaces the entire child process environment instead of merging user-specified keys with the inherited environment.

Problem

When a .NET embedder sets Environment to augment PATH or add a few variables, the SDK calls startInfo.Environment.Clear() (Client.cs:1151), wiping all inherited platform variables (COMSPEC, SystemRoot, TEMP, LOCALAPPDATA, etc.). On Windows this breaks ConPTY shell spawning — the CLI's PowerShell tool fails with File not found: (empty path) because the shell executable can't be resolved without these variables.

Before (broken):

if (options.Environment != null)
{
    startInfo.Environment.Clear();       // <-- wipes ALL inherited vars
    foreach (var entry in options.Environment)
        startInfo.Environment[entry.Key] = entry.Value;
}

Fix

Remove the Environment.Clear() call. In .NET, ProcessStartInfo.Environment is pre-populated with the current process's environment, so iterating through the user-provided keys naturally merges (overrides specified keys, inherits the rest).

After (fixed):

if (options.Environment != null)
{
    foreach (var entry in options.Environment)
        startInfo.Environment[entry.Key] = entry.Value;  // merge: override or add
}

Cross-SDK Comparison

SDK	Behavior when user provides env	Notes
Node.js	Merge ({ ...process.env, ...options.env })	Fixed in this PR to match .NET
Python	Merge (dict(os.environ); env.update(cfg.env))	Fixed in this PR to match .NET
Go	Full replacement (options.Env or os.Environ())	Documented: callers must pass append(os.Environ(), ...)
.NET (this PR)	Merge — override specified keys, inherit rest	Natural for .NET: ProcessStartInfo.Environment starts pre-populated

Testing

• All 157 .NET tests pass (6 skipped for unrelated reasons)
• 9 new EnvironmentTests prove merge semantics via public API only (StartAsync + PingAsync)
• 6 new Node.js env-option tests confirm merge semantics
• Existing E2E test harness already copies full environment (E2ETestContext.GetEnvironment()), so behavior is unchanged for existing callers
• No breaking change: callers providing a full env dict get the same result. Callers providing a partial dict now get correct merge behavior instead of a broken stripped environment.

References

• Bug report: PureWeen/PolyPilot#441 — filed in the PureWeen/PolyPilot repo, not github/copilot-sdk
• Related: PureWeen/PolyPilot#442 (downstream symptom: empty shell path in ConPTY)

[PureWeen]

Closing in favor of a clean PR from fix/env-merge-semantics that contains only the env-fix changes (no session.idle contamination).