fix(microsandbox): mount workspace source and make it usable by the remote user

[skevetter]

Summary

Fixes the microsandbox microVM backend so a devcontainer workspace actually works. Previously VS Code failed to connect: the workspace source was never shared into the microVM, so /workspaces/<name> did not exist ("Workspace does not exist"), and even once mounted the virtiofs share was root-owned and unusable by a non-root remote user.

Three root causes addressed, plus an OOM default:

1. Workspace source not mounted. The driver ignored options.WorkspaceMount and dropped bind mounts entirely. Now the workspace bind mount and any MountTypeBind mounts are translated to msb --mount-dir SOURCE:DEST[:ro] (virtiofs).
2. Mount root-owned in the guest. msb's virtiofs share presents files as root:root with no UID mapping, and microsandbox does no UID remap. Added a driver.WorkspaceChowner capability; microsandbox reports it, so the agent recursively chowns the workspace to the remote user — the same treatment podman machine already gets for the identical problem.
3. ChownR aborted early. The recursive chown bailed on the first un-chownable entry (read-only .git pack files on the share), leaving everything after it root-owned. It now skips failed entries, finishes the walk, and returns aggregated errors.
4. OOM on connect. MICROSANDBOX_MEMORY now defaults to 2048 MiB, so the VS Code extension host no longer gets OOM-killed on first connect (the VM previously booted at the runtime's ~512 MiB default). User-set values still win.

Validation

Verified end-to-end on a clean microsandbox workspace (cross-compiled Linux agent injected via DEVSY_AGENT_BINARY):

• msb inspect shows the /workspaces/<name> bind mount.
• Setup log shows chown workspace recursively: user=vscode.
• Over SSH and msb exec as vscode: cd into the folder works, all files (incl. app.py, .gitignore, .github/*) are vscode-owned and editable, git status clean.

gofmt, go vet, and all tests in the touched packages (267) pass.

Notes

• The agent-side fixes (ChownR, chownWorkspace gating) ship in the Linux agent binary and take effect once a release is built from this code.
• The memory default only applies to freshly-resolved provider options; existing workspaces with an explicit value are unchanged.

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📥 Commits

Reviewing files that changed from the base of the PR and between 9776e9d and 5dc8b6d.

📒 Files selected for processing (11)

• pkg/copy/copy.go
• pkg/devcontainer/setup.go
• pkg/devcontainer/setup/setup.go
• pkg/devcontainer/setup_test.go
• pkg/driver/microsandbox/cliclient.go
• pkg/driver/microsandbox/cliclient_test.go
• pkg/driver/microsandbox/client.go
• pkg/driver/microsandbox/microsandbox.go
• pkg/driver/microsandbox/microsandbox_test.go
• pkg/driver/types.go
• providers/microsandbox/provider.yaml

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