fix(sessions): preserve native Bash failure status in previews

[ooiuuii]

Problem

Fixes #575.

Native Pi bashExecution messages carry exitCode and cancelled, not the isError field alone used by the Sessions preview. Failed commands can therefore appear without a failure marker in history.

Value

Preserve the command's actual failure or cancellation outcome when reviewing a past session.

Approach

Project native cancellation and defined nonzero exit codes onto the existing Bash preview error flag. Preserve explicit legacy isError; keep unknown exit status unmarked. No execution or canonical-history changes.

Validation

• Actual isolated SDK AgentSession.executeBash("exit 7") produces the canonical message used by the preview regression; no model/provider call or private state.
• New tests: unchanged code 3 pass / 4 fail; fixed 7/7 pass.
• Complete Sessions group: 42/42 Node, 42/42 Bun. Covers success, nonzero exit, cancellation, unknown status, legacy flags and unchanged history.
• bun run check, git diff --check, one scoped read-only review — pass/no actionable findings.
• Full local suite not claimed green: this host has separately reproduced baseline timing failures and Windows child-cleanup hangs. The unsafe group was not repeated; fresh branch CI supplies broader validation.

Impact

• User-visible: accurate Bash failure/cancellation marker in Sessions previews.
• Model-visible/tools/runtime execution: unchanged.
• Persisted config/data: unchanged.
• Compatibility/risk: existing preview shape and sanitization retained; no new status policy, shell behavior, or session lifecycle.