feat: Add Python SAST workflow with three security analysis tools

Implements Issue #5 - Python SAST workflow that combines:
- Dependency scanning (pip-audit) for CVE detection
- Security linting (Bandit) for vulnerability patterns
- Type checking (Mypy) for type safety issues

## Changes

**New Modules:**
- `DependencyScanner`: Scans Python dependencies for known CVEs using pip-audit
- `BanditAnalyzer`: Analyzes Python code for security issues using Bandit
- `MypyAnalyzer`: Checks Python code for type safety issues using Mypy

**New Workflow:**
- `python_sast`: Temporal workflow that orchestrates all three SAST tools
  - Runs tools in parallel for fast feedback (3-5 min vs hours for fuzzing)
  - Generates unified SARIF report with findings from all tools
  - Supports configurable severity/confidence thresholds

**Updates:**
- Added SAST dependencies to Python worker (bandit, pip-audit, mypy)
- Updated module __init__.py files to export new analyzers
- Added type_errors.py test file to vulnerable_app for Mypy validation

## Testing

Workflow tested successfully on vulnerable_app:
- ✅ Bandit: Detected 9 security issues (command injection, unsafe functions)
- ✅ Mypy: Detected 5 type errors
- ✅ DependencyScanner: Ran successfully (no CVEs in test dependencies)
- ✅ SARIF export: Generated valid SARIF with 14 total findings

workers/python/requirements.txt

@@ -16,3 +16,8 @@ a2a-sdk[all]>=0.1.0
 
 # Fuzzing
 atheris>=2.3.0
+
+# SAST Tools
+bandit>=1.7.0
+pip-audit>=2.6.0
+mypy>=1.8.0