fix: .NET Environment.Clear() wipes inherited env vars + cross-SDK merge alignment

[PureWeen]

Problem

Fixes PureWeen/PolyPilot#441.

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.

When a .NET embedder sets Environment to augment PATH or add a few variables, the SDK calls startInfo.Environment.Clear() (Client.cs), 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
}

This PR also aligns Node.js and Python to the same merge semantics, and documents Go's different behavior (full replacement by design).

Cross-SDK Comparison

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

Testing

• 9 new EnvironmentTests in .NET prove merge semantics via public API only (StartAsync + PingAsync)
• 6 new Node.js env-option tests confirm merge semantics
• All existing tests continue to pass
• 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: External: copilot-sdk Environment.Clear() replaces child process environment instead of merging PolyPilot#441 — filed in the PureWeen/PolyPilot issue tracker
• Related: External: copilot-cli shell tool 'File not found' error provides no diagnostic information PolyPilot#442 (downstream symptom: empty shell path in ConPTY)