feat: migrate to capabilities, add build-in guardrails, rename from pydantic-ai-middleware to pydantic-ai-shields

[DEENUU1]

[0.3.0] - 2026-03-28

Changed

• Renamed to pydantic-ai-shields — the package is now focused exclusively on guardrail capabilities built on
  pydantic-ai's native capabilities API. The old middleware layer (MiddlewareAgent, AgentMiddleware, etc.) has been
  removed — pydantic-ai v1.71+ provides this natively.

Added

• 5 infrastructure capabilities:
  • CostTracking — token usage tracking, USD cost calculation via genai-prices, budget enforcement
  • ToolGuard — block tools via prepare_tools or require approval via before_tool_execute
  • InputGuard — block/check user input with pluggable guard function (sync or async)
  • OutputGuard — block/check model output with pluggable guard function (sync or async)
  • AsyncGuardrail — run guardrail concurrently with LLM call, 3 timing modes
• 5 built-in content shields (zero external dependencies):
  • PromptInjection — detect prompt injection / jailbreak across 6 categories with 3 sensitivity levels
  • PiiDetector — detect PII (email, phone, SSN, credit card, IP) with block or log action
  • SecretRedaction — block API keys, tokens, credentials in model output (OpenAI, Anthropic, AWS, GitHub,
    Slack, JWT, private keys)
  • BlockedKeywords — block forbidden keywords/phrases with case, whole-word, and regex modes
  • NoRefusals — block LLM refusals with 10 built-in patterns and partial refusal support

Removed

• All legacy middleware modules: MiddlewareAgent, AgentMiddleware, MiddlewareToolset, MiddlewareChain,
  MiddlewareContext, ConditionalMiddleware, ParallelMiddleware, AsyncGuardrailMiddleware,
  CostTrackingMiddleware, decorators, config loaders, pipeline spec, builder
• All legacy examples and documentation