fix: surface top-level metadata on StepResult in typed env clients

[sergiopaniego]

Summary

Follow-up to #805. That PR surfaced a top-level metadata field on StepResult and updated the generic, MCP, and template clients, but the 20 typed env clients (envs/*/client.py) still built StepResult
without it, so result.metadata was None even when the observation carried metadata. This applies the same pattern to every typed client's _parse_result.

Type of Change

• Bug fix

Alignment Checklist

Before submitting, verify:

• I have read .claude/docs/PRINCIPLES.md and this PR aligns with our principles
• I have checked .claude/docs/INVARIANTS.md and no invariants are violated
• I have run /pre-submit-pr (or bash .claude/hooks/lint.sh and tests) and addressed all issues

RFC Status

• Not required (bug fix, docs, minor refactoring)

Test Plan

Each _parse_result now mirrors the pattern merged in #805:

• metadata=payload.get("metadata", obs_data.get("metadata", {})) on the Observation
• metadata=payload.get("metadata") on the returned StepResult

Backwards compatible: when the server sends no top-level metadata (older servers), it falls back to the nested observation.metadata, so existing behavior is unchanged. Verified all 20 files compile and pass
ruff format. Affected envs: atari, browsergym, chat, chess, calendar, connect4, finrl, dm_control, maze, openapp, reasoning_gym, kernrl, snake, repl, tbench2, openspiel, textarena, websearch, sumo_rl, unity.

Claude Code Review

N/A

[bot-ci-comment]

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