feat: Add Pillar Security guardrail integration (BerriAI#12791)

* feat: Add Pillar Security guardrail integration

Implements comprehensive LLM security guardrails using Pillar Security API with support for prompt injection detection, PII/secret detection, content moderation, and multi-mode execution (pre_call, during_call, post_call). Includes complete documentation, testing, and configurable actions on flagged content.

* fix: Resolve MyPy type error in Pillar guardrail config

Restructure PillarGuardrailConfigModel to properly inherit from GuardrailConfigModel[T]
and resolve return type compatibility issue in get_config_model method.

* fix: Resolve MyPy type error in Pillar guardrail config

Restructure PillarGuardrailConfigModel to properly inherit from GuardrailConfigModel[T]
and resolve return type compatibility issue in get_config_model method.

* fix docs

* fix docs

* improved docs

* fix examples, READY

Author: eagle-p