Add direct dev environment connections

[wolegechu]

Summary

• add a public Run.get_direct_connection() resolver and validated RunDirectConnection argument vectors for running, owner-accessible dev environments
• add dstack connect for direct SSH or configured VS Code, Cursor, Windsurf, and Zed access through SSHProxy without a resident attach process
• fail closed on missing or inconsistent proxy/IDE metadata, validate the complete record before local SSH state changes, and never invoke a shell or fall back to direct host SSH
• document the CLI, Python API, SSHProxy requirements, and the boundary from dstack attach

Part of #4123.

CLI transcript

$ dstack connect --help
Usage: dstack connect [-h] [--project NAME]
                      [--ssh | --ide | --vscode | --cursor | --windsurf | --zed]
                      [--replica REPLICA] [--job JOB] [--expect-sshproxy HOST[:PORT]]
                      run_name
...
  --zed                 Open the dev environment in Zed
  --expect-sshproxy HOST[:PORT]
                        Fail unless the run uses this exact SSH proxy endpoint

$ dstack connect --ssh --zed dev-run
dstack connect: error: argument --zed: not allowed with argument --ssh

Validation

• pytest -q src/tests/api/test_runs.py src/tests/api/test_direct_connect.py src/tests/_internal/cli/commands/test_connect.py (57 passed)

• ruff format --check on all changed Python files

• ruff check on all changed Python files

• git diff --check

• Pyright with the project virtual environment: 0 errors, 0 warnings

• pre-commit for the PR diff: passed

• mkdocs build --strict: passed

AI assistance

This PR was developed primarily with AI assistance from OpenAI Codex. I directed the design, reviewed the resulting diff, and ran the listed validations.