[Python] Fix exception variable captured in deferred closures (NameError)

[dbrattli]

Summary

• Python's except E as name: implicitly deletes name at end of the except block (PEP 3110). Closures that capture the variable and are called after the block exit get a NameError: free variable 'ex' referenced before assignment in enclosing scope.
• Fix in makeHandler (transformTryCatch): copy ex → ex_ immediately inside the block via an annotated assignment, then rename all body references using FableTransforms.replaceValues. The original name stays in except ... as ex: for readability.
• Handles both zero-arg closures (fun () -> ex.Message) and lambda-with-argument patterns (fun obv _ -> obv.OnErrorAsync ex) — the latter mirrors the AsyncRx defer function.

Before:

except Exception as ex:
    def _arrow107(obv, _arg):
        return obv.OnErrorAsync(ex)  # NameError: ex deleted by Python
    result = of_async_worker(_arrow107)

After:

except Exception as ex:
    ex_: Exception = ex             # safe copy, Python won't delete this
    def _arrow107(obv, _arg):
        return obv.OnErrorAsync(ex_)  # uses safe copy
    result = of_async_worker(_arrow107)

Test plan

• Two new regression tests in TestMisc.fs covering both closure patterns
• All 2080 Python tests pass (./build.sh test python --skip-fable-library)
• Verified fix in AsyncRx defer function output

🤖 Generated with Claude Code

[github-actions]

Python Type Checking Results (Pyright)

Metric	Value
Total errors	18
Files with errors	4
Excluded files	4
New errors	✅ No

Excluded files with errors (4 files)

These files have known type errors and are excluded from CI. Remove from pyrightconfig.ci.json as errors are fixed.

File	Errors	Status
temp/tests/Python/test_applicative.py	12	Excluded
temp/tests/Python/test_hash_set.py	3	Excluded
temp/tests/Python/test_nested_and_recursive_pattern.py	2	Excluded
temp/tests/Python/fable_modules/thoth_json_python/encode.py	1	Excluded