resolveSandboxedMediaSource() rejected all paths outside the sandbox
workspace root, including /tmp. This blocked sandboxed agents from
sending locally-generated temp files (e.g. images from Python scripts)
via messaging actions.
Add an os.tmpdir() prefix check before the strict sandbox containment
assertion, consistent with buildMediaLocalRoots() which already
includes os.tmpdir() in its default allowlist. Path traversal through
/tmp (e.g. /tmp/../etc/passwd) is prevented by path.resolve()
normalization before the prefix check.
Relates-to: #16382, #14174
