feat(script-engines): implement complete Python engine for Issue #253

- Add PythonEngine with subprocess execution and timeout handling
- Implement context injection with JSON serialization and Python syntax conversion
- Add comprehensive error handling for syntax, runtime, import, and timeout errors
- Integrate memory tracking and performance monitoring capabilities
- Support script precompilation (source storage) and execution
- Add structured stderr log parsing for debugging capabilities
- Add 13 comprehensive tests covering all functionality:
  * Engine creation and configuration
  * Simple script execution and context injection
  * Error handling (syntax, runtime, import errors)
  * Timeout enforcement and memory tracking
  * MCP validation script support
  * Performance requirements validation (<1000ms for concurrent execution)
  * Concurrent execution support
- All tests passing with proper error handling and resource management

Design document: docs/design/issue-253-python-engine-implementation.md
Tests: 13 tests, all passing (coverage: comprehensive)
Dependencies: Add 'which' crate for Python interpreter detection

closes #253

Author: milliondreams

crates/mandrel-mcp-th/Cargo.toml

@@ -48,6 +48,9 @@ tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 jsonpath_lib = "0.3"
 jsonschema = "0.18"
 
+# Script engine dependencies  
+which = "4.0"
+
 # Utilities
 uuid = { version = "1.0", features = ["v4", "serde"] }
 chrono = { version = "0.4", features = ["serde"] }