feat: Add Python exception classes for structured error handling (Pha…

[shaia]

…se 4)

Add exception hierarchy for CFD error codes with proper Python integration:

• CFDError base class with status_code and message attributes
• CFDMemoryError, CFDInvalidError, CFDIOError inherit from builtin types
• CFDUnsupportedError, CFDDivergedError, CFDMaxIterError for solver errors
• raise_for_status() helper to convert status codes to exceptions

[Copilot]

Pull request overview

This PR implements a comprehensive Python exception hierarchy for structured error handling in the CFD library, providing type-safe error propagation and integration with Python's built-in exception system.

Key Changes:

• Added CFDError base exception class with status_code and message attributes for all CFD-related errors
• Created specialized exception classes (CFDMemoryError, CFDInvalidError, CFDIOError, CFDUnsupportedError, CFDDivergedError, CFDMaxIterError) with appropriate inheritance from both CFDError and built-in Python exception types
• Implemented raise_for_status() helper function to automatically convert CFD status codes into appropriate exceptions with context-aware error messages

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
cfd_python/_exceptions.py	New module defining the complete exception hierarchy with 7 exception classes, status-to-exception mapping, and the raise_for_status() helper function
cfd_python/__init__.py	Imports and exports all exception classes and raise_for_status() function in the public API
tests/test_errors.py	Comprehensive test suite covering exception class attributes, inheritance patterns, raise_for_status() behavior for all error codes, and proper module exports
MIGRATION_PLAN.md	Updates Phase 4 section to reflect completion status, implementation details, and actual effort (<0.5 days)

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