Add Thai/Vietnamese injection detection patterns

[Acacian]

Summary

The injection guardrail supports Korean, Chinese, and Japanese patterns, but Thai and Vietnamese are missing. These are important for multi-language coverage.

What to do

1. Open src/aegis/guardrails/injection.py
2. Add patterns for Thai and Vietnamese injection attempts in the multi_language category:
   • "Ignore previous instructions" equivalents
   • "You are now" role-change equivalents
   • "Show system prompt" extraction equivalents
3. Add tests in tests/test_guardrails_injection.py

Context

Look at the existing Korean (ko_*) and Chinese (zh_*) patterns in INJECTION_PATTERNS['multi_language'] for the pattern to follow. Each pattern needs: name, regex, confidence, and sensitivity fields.

Acceptance criteria

• At least 3 Thai patterns and 3 Vietnamese patterns added
• All existing tests still pass
• New test cases for each added pattern