revert: restore single-session MCP client behavior

[cursor]

Summary

Reverts #1175, which enables production HTTP MCP alongside the existing WebSocket lifecycle without a shared session identity. Default-capacity servers can reject connect() when the second session is created; when capacity permits, reset/step/state and tool calls operate on different environment instances.

This restores src/openenv/core/mcp_client.py and its mode-selection tests exactly to #1175's parent tree while the shared-session design is resolved through RFC/transport review.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation
• New environment
• Refactoring

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
• The revert tree was validated exactly; CI will rerun on this branch

RFC Status

• Not required (revert of an unreviewed lifecycle regression)
• RFC exists: #___
• RFC needed (will create before merge)

Test Plan

• Verified the revert commit tree for the two affected files is byte-identical to fix(mcp): production JSON-RPC routing with sync-safe teardown (from #1169) #1175's parent 08af2d2f.
• git diff --check origin/main...HEAD passes.
• Require exact-head Tests, Package CI, lint, and Bugbot before merge.

Claude Code Review

Bugbot independently reports that #1175 opens two environment sessions and fails when max_concurrent_envs=1. The revert is the minimal release-safe action; a future shared-session implementation needs explicit RFC 003/transport-owner review and stateful async/sync E2E coverage.