Pipeline validation for AI filter composition

[shaneutt]

Validate filter ordering and compatibility for AI inference pipelines. Catch misconfiguration at startup rather than at runtime.

Requirements

• Validate AI filter ordering constraints (e.g. credential_injection before provider routing)
• Detect missing required filters (e.g. model_to_header needed before model-based routing)
• Validate provider feature flag availability matches configured providers
• Warn on redundant filter combinations
• Clear error messages with suggested corrections

Acceptance Criteria

• Invalid pipeline rejected at config load with actionable error message
• Valid pipelines pass validation without false positives
• Unit tests for each validation rule
• Integration test with example configs