Add fault injection support to proxy backend

[epli2]

Summary

This PR adds comprehensive fault injection capabilities to the proxy backend, allowing users to inject artificial delays and synthetic HTTP errors into proxied requests via CLI flags. This enables testing of client resilience and error handling without modifying the actual backend services.

Key Changes

• New fault module (crates/phantom-capture/src/fault.rs):

  • Defines FaultRule enum supporting two injection types:
    • Delay: Inject fixed or random latency (with optional URL pattern matching)
    • Error: Return synthetic HTTP error responses with configurable probability
  • Implements FaultConfig to hold an ordered list of rules
  • Provides parse_fault_spec() parser supporting flexible CLI syntax:
    • delay:100ms — fixed delay
    • delay:100ms-500ms — random range
    • delay:200ms:/api — conditional on URL pattern
    • error:503 — always inject error
    • error:500:0.5:/api — probabilistic error with URL filter
  • Includes comprehensive unit tests for all parsing scenarios

• Updated ProxyCaptureBackend (crates/phantom-capture/src/proxy.rs):

  • Added fault_config field to store fault rules
  • Implemented with_faults() builder method for configuration
  • Integrated fault rule evaluation in handle_request():
    • Applies delays via tokio::time::sleep()
    • Intercepts requests matching error rules and returns synthetic responses
    • Emits proper HttpTrace records for injected faults
    • Uses random number generation for probabilistic injection

• CLI integration (src/main.rs):

  • Added --fault SPEC flag (repeatable for multiple rules)
  • Implemented build_fault_config() to parse and validate fault specifications
  • Integrated fault config into proxy backend initialization
  • Added comprehensive help text with examples

• Module exports (crates/phantom-capture/src/lib.rs):

  • Exported FaultConfig, FaultRule, and parse_fault_spec for public use

Implementation Details

• Fault rules are evaluated in order during request handling
• URL pattern matching uses simple substring containment (no regex)
• Delays use rand::random() for range selection
• Error injection short-circuits normal request forwarding and immediately emits a trace
• Injected error responses include x-fault-injected: phantom header for identification
• All parsing errors provide clear, actionable error messages

https://claude.ai/code/session_013JsZoVqikbBRzwiLjDozKM