feat(sandbox): forward capability options through Sandbox.create (#284)

Add the optional BashOptions capability flags (python, javascript, commands,
customCommands, fetch) to SandboxOptions and forward them into the internal
new Bash({...}) that Sandbox.create builds. They were previously dropped, so a
host driving just-bash via the Sandbox API could not enable python3, js-exec, a
restricted command set, custom commands, or a custom fetch even though the
runtimes ship in the package.

Behavior is unchanged when the fields are omitted (each falls back to its
existing BashOptions default). Adds forwarding tests in the unit suite
(customCommands + restricted commands) and a python3 end-to-end test in the
WASM suite, plus a changeset.

Signed-off-by: Ari Mayer <ari111097@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>