Fix ObjectDisposedException when disposing session after client.StopAsync()

[parthtrivedi2492]

Summary

Fixes #306

When await client.StopAsync() is called before the await using var session goes out of scope, session.DisposeAsync() is called twice:

1. First by StopAsync() (while connection is alive) ✓
2. Second by await using cleanup (connection is now disposed) ✗ → ObjectDisposedException

Repro (from issue)

using GitHub.Copilot.SDK;

await using var client = new CopilotClient();
await using var session = await client.CreateSessionAsync();

await client.StopAsync();
// When method exits, 'await using var session' tries to dispose → CRASH

Solution

Make CopilotSession.DisposeAsync() idempotent and tolerant to already-disposed connections:

1. Added _isDisposed flag - Thread-safe using Interlocked.Exchange
2. Made DisposeAsync idempotent - Returns immediately if already disposed (required by IAsyncDisposable contract)
3. Handle connection failures gracefully - Catches ObjectDisposedException and IOException since dispose methods should never throw

This follows the same pattern already used in CopilotClient.DisposeAsync().

Changes

• dotnet/src/Session.cs: Add idempotency and exception handling to DisposeAsync()
• dotnet/test/ClientTests.cs: Add regression test

Testing

• All 13 ClientTests pass including the new regression test
• Build passes with no errors

[Copilot]

Pull request overview

This PR fixes a double disposal bug where disposing a CopilotSession after calling client.StopAsync() would throw an ObjectDisposedException. The issue occurred because StopAsync() disposes all sessions, and then the await using cleanup would attempt to dispose the session again on an already-closed connection.

Changes:

• Added thread-safe idempotency to CopilotSession.DisposeAsync() using an _isDisposed flag with Interlocked.Exchange
• Added exception handling to gracefully handle disposal when the connection is already disposed
• Added a regression test to prevent this issue from recurring

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
dotnet/src/Session.cs	Implements idempotent disposal with thread-safe flag and exception handling for ObjectDisposedException and IOException
dotnet/test/ClientTests.cs	Adds regression test that reproduces the issue scenario: creating a session, calling StopAsync(), then allowing implicit disposal