Implement proper try/except/finally handling in TIFA visitor

[Copilot]

TIFA (Type Inference and Flow Analysis) was not correctly handling try/except/finally constructs, leading to missed variable initialization issues. The visitor used the default generic_visit method which doesn't properly track control flow through exception handling blocks.

Problem

Consider this code:

try:
    x = 1
except:
    pass  # x not defined in this branch
print(x)  # Should be flagged as potentially uninitialized

Previously, TIFA would not detect that x might be uninitialized because it wasn't analyzing the different execution paths through try/except blocks.

Solution

Added proper AST visitor methods for try/except/finally constructs:

• visit_Try: Handles the main try statement by creating separate execution paths for:

  • Try body (including else clause if no exception occurs)
  • Each exception handler
  • Finally clause (always executes)

• visit_except_handler: Handles individual except clauses including:

  • Exception type checking
  • Exception binding (except Exception as e:)
  • Exception handler body execution

The implementation properly merges all possible execution paths to ensure variable initialization is tracked across all branches.

Key Features

• ✅ Detects variables that may be uninitialized in some execution paths
• ✅ Handles exception binding with as clause
• ✅ Supports else and finally clauses
• ✅ Works with nested try/except constructs
• ✅ Maintains compatibility with existing TIFA functionality

Testing

Added comprehensive test cases covering:

• Variables defined in both try and except blocks
• Variables defined only in try block
• Variables defined only in except block
• Finally clause variable definitions
• Exception binding scenarios

All tests pass and no regressions were introduced (526 total tests, same 16 expected failures as baseline).

Fixes #157.

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