Add session.extensions.sendAttachmentsToMessage SDK wrapper

Regenerates the SDK clients to expose the new RPC endpoint added in
copilot-agent-runtime PR #8971, which lets canvas extensions push
attachments (including a new extension_context variant) into the next
user message's composer.

Languages regenerated: TypeScript, .NET, Python, Rust, Java.

Go is intentionally not regenerated in this PR — the Go codegen has a
duplicate-method bug when a leaf type is shared across two discriminated
unions with the same discriminator property (SendAttachment and
PushAttachment both use 'type'), causing Type() to be emitted twice.
Tracking as follow-up.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jmoseley