fix: escape command block payload to prevent prompt injection

[glasscatya]

Problem Description

If command output contains </command> (or similar tag-like payload), it can break command-block boundaries and
create a prompt-injection risk in follow-up context.

Resolution

Added escaping for command-block payload and expanded unit test coverage.

• Escaped command block content in CommandExecutionResult.block():
  • escape name and status as attributes
  • escape output as text content
• Added tests for injection-like payload in command output:
  • assistant command follow-up path
  • user command error fallback path

Verification

• ruff check
• ruff format
• mypy
• pre-commit