Close Language Gaps for SessionFs

[j1m-ryan]

Adds SessionFs virtual file system support to the .NET, Go, and Python SDKs.

[Copilot]

Pull request overview

Adds SessionFs (virtual/session-scoped filesystem) support across the Python, Go, and .NET SDKs so the client can register as a filesystem provider and the CLI can call back into the SDK for session file I/O.

Changes:

• Introduces SessionFs config + handler interfaces and wires JSON-RPC request handling for sessionFs.* methods in Python/Go/.NET clients.
• Extends session creation/resume APIs to require a per-session SessionFs handler factory when SessionFs is enabled at the client level.
• Adds unit + E2E coverage for SessionFs behavior (routing writes through provider + resuming from provider state).

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
python/test_client.py	Unit tests for Python SessionFs config storage + missing handler validation.
python/e2e/test_session_fs.py	Python E2E coverage using an in-memory SessionFs handler and replay snapshots.
python/copilot/session.py	Stores per-session SessionFs handler on CopilotSession.
python/copilot/client.py	Adds SessionFsConfig/SessionFsHandler, registers provider, and handles sessionFs.* RPC callbacks.
python/copilot/init.py	Exports SessionFs types from the package.
go/types.go	Adds Go SessionFsConfig + SessionFsHandler and handler factory fields on session configs.
go/session.go	Stores per-session SessionFs handler on Session.
go/internal/e2e/session_fs_test.go	Go E2E coverage using an in-memory SessionFs handler and replay snapshots.
go/client.go	Registers SessionFs provider and implements sessionFs.* JSON-RPC request handlers.
go/client_test.go	Unit tests for Go SessionFs options + missing handler validation.
dotnet/test/SessionFsTests.cs	.NET unit tests for SessionFs config + handler factory cloning behavior.
dotnet/test/SessionFsE2ETests.cs	.NET E2E coverage using an in-memory SessionFs handler and replay snapshots.
dotnet/src/Types.cs	Adds .NET SessionFs types and JSON source-gen registrations; extends session configs.
dotnet/src/Session.cs	Stores per-session SessionFs handler and routes sessionFs.* requests to it.
dotnet/src/Client.cs	Registers provider on connect; enforces handler factory requirement; wires RPC handlers.

[j1m-ryan]

Closing because of #1036

[SteveSandersonMS]

Thanks for submitting this, @j1m-ryan!

I started by thinking we'd use this, but then realised we needed to add a load of new codegen logic to C#/Python/Go, and by the time I had done that Copilot finished off most of the rest of it.

So I hope you don't mind the duplication of work. It was easier to complete by letting Copilot get to the end of the process on its own.

[j1m-ryan]

Not at all, thank you @SteveSandersonMS, we are glad its being worked on by the team