cli: avoid shell=True for npx/browser helpers

[TheodorNEngoy]

This removes shell=True usage from the CLI helper path (a common security footgun).

Changes:

• _get_npx_command() now returns a subprocess-friendly command prefix and uses COMSPEC /c only when npx is a .cmd/.bat shim.
• mcp dev runs npx @modelcontextprotocol/inspector without shell=True.
• URL elicitation example uses webbrowser.open() for cross-platform browser opening.

Tests:

• uv run pytest -q tests/cli/test_utils.py
• uv run ruff check src/mcp/cli/cli.py tests/cli/test_utils.py examples/snippets/clients/url_elicitation_client.py