fix(tui-gateway): expose cli.exec stdout and stderr as separate fields

[Jabberwocky1989]

What does this PR do?

cli.exec returns a single output string: stdout and stderr joined with a newline. A machine
consumer that parses stdout as structured data therefore breaks as soon as anything writes to
stderr — a command emitting valid JSON on stdout plus a few warning lines on stderr yields an
output that cannot be parsed, even though the command exited 0.

This adds stdout and stderr as separate fields. output is unchanged and remains
byte-identical at its default bound, so existing callers are unaffected.

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• tui_gateway/methods_tools.py: new pure helper _cli_exec_output(r, limit=48_000) returning
  output (unchanged semantics), stdout, and stderr; the existing non-blocked cli.exec
  branch builds its payload from it.
• tests/tui_gateway/test_cli_exec_streams.py: focused tests for split streams, empty/None
  streams, per-field bounds incl. default-limit output identity, and the exact key set.

The blocked branch is untouched and still returns blocked, hint, code, output.

How to Test

1. scripts/run_tests.sh tests/tui_gateway/test_cli_exec_streams.py → 4 passed.
2. Drive the registered handler (server._methods["cli.exec"]) with a real subprocess:
   argv ["--help"] → code 0, non-empty stdout; an unknown subcommand → non-zero code,
   non-empty stderr; argv [] → the unchanged blocked payload.

Exposure and interaction with open work

• The new fields are additive and each independently bounded at 48,000 characters. Combined they
  can surface up to ~96,000 characters, and stderr can carry bytes that the combined output
  currently drops once stdout fills that budget. This is a widening, not merely a re-split.
• No redaction is applied to this payload on the base. Open PR fix(tui-gateway): redact secrets from exec RPC output and guard command.dispatch #45794 adds redaction to this RPC's
  output and should cover the new fields when it lands.
• Neighbouring handler code is under active change (fix(tui-gateway): redact secrets from exec RPC output and guard command.dispatch #45794, fix(security): route tui_gateway shell.exec through the sanitized env builder #78036, fix(tui_gateway): scrub credentials from shell.exec subprocess env #60423), so a conflict is
  possible; this change touches neither env nor credential handling.
• Tested on Windows 11.