fix(gateway): bridge terminal.docker_extra_args to env for gateway sessions (#30415)

[Tranquil-Flow]

What does this PR do?

Fixes #30415 — adds the missing docker_extra_args entry to gateway/run.py's _terminal_env_map, bridging the terminal.docker_extra_args config key to the TERMINAL_DOCKER_EXTRA_ARGS env var.

Why is this needed?

config.yaml's terminal.docker_extra_args is silently dropped when the gateway spawns per-session sandbox containers. Every other terminal.docker_* key in that block IS bridged, so users reasonably expect this one to be too. Net effect: sandbox containers spawn without documented extra args (bind mounts, devices, capabilities).

Root cause

The _terminal_env_map in gateway/run.py omits docker_extra_args. The existing list/dict serialization logic (lines 482-485) already handles the encoding — only the map entry was missing.

How to test

1. Set terminal.docker_extra_args in config.yaml
2. Run via gateway with docker backend
3. Verify the argument reaches the spawned container

Existing env-bridge test passes.

Checklist

• Bug fix (non-breaking change)
• One logical change per PR
• Follows codebase style
• No new dependencies
• CONTRIBUTING.md guidelines followed

[alt-glitch]

Duplicate of #30629 (closed, same fix). Superseded by #28891 which is the superset fix bridging both docker_extra_args AND docker_forward_env to all three config-env maps. This PR only covers one map entry.

[benbarclay]

Closing as obsolete — the terminal.docker_* config→env bridge this adds is already complete and test-pinned on main. Both bridge maps carry the docker keys (cli.py env_mappings and gateway/run.py _terminal_env_map), and tests/tools/test_terminal_config_env_sync.py pins them via test_docker_extra_args_is_bridged_everywhere / test_docker_env_is_bridged_everywhere / test_docker_forward_env_is_bridged_everywhere / test_docker_volumes_is_bridged_everywhere (all passing). So terminal.docker_extra_args / docker_env / docker_forward_env set in config.yaml already reach TERMINAL_DOCKER_* for both CLI/TUI and gateway sessions. Thanks for the contribution — the gap it targeted has since been closed in-tree.

(Note for maintainers: the consumer-side gap where code_execution/file_tools sandboxes drop docker_env/docker_extra_args is a separate issue, tracked via #35937 — not closed here.)