feat(python): give System exceptions native Python bases

[dbrattli]

Python except TimeoutError currently misses a generated .NET TimeoutException. The same interoperability gap applies to other .NET exceptions with a natural Python counterpart. This change keeps distinct .NET exception classes while making Python's builtin handlers catch them.

Changes

• Copy System.fs into the Python runtime and update its project reference.
• Centralize ExceptionBase and the native bases in exception_bases.py. Keep types.ExceptionBase as a re-export of the same class for existing generated code and Python imports; new compiler output imports the canonical module.
• Add native bases that retain ExceptionBase ancestry, message formatting, and inner_exception while also inheriting the corresponding Python builtin.
• Preserve the existing .NET reflection ancestry by hiding the native bridge classes in generated reflection metadata.
• Add F# regressions for the mappings, specific .NET catches, payloads, native timeout catches, reflection, and identity across both ExceptionBase import paths.

.NET exception	Python base
ArgumentException and its subclasses, FormatException	ValueError
IndexOutOfRangeException	IndexError
InvalidOperationException	RuntimeError
DivideByZeroException	ZeroDivisionError
OverflowException	OverflowError
NotImplementedException	NotImplementedError
OutOfMemoryException	MemoryError
TimeoutException	TimeoutError

Related to the Python exception compatibility discussion in #5035. Existing async entry points keep their current exception contracts.

Validation

• Full .NET Python suite: 2,399 passed.
• Full generated Python suite on Python 3.14: 2,493 passed, with two warnings from existing cancellation timer tests.
• Full generated Pyright reports 34 errors / 45 warnings, with no increase from the existing baseline. The changed hand-written runtime files have no diagnostics.
• The native mapping and timeout-handler regressions fail against main's runtime and pass with this change. Confirmed the reflection bridge interrupted ancestry before the generator adjustment.
• All 43 constructor exports and signatures match main.
• Targeted inheritance/payload probes pass on Python 3.12.13, 3.13.8, and 3.14.0.
• Compiler/runtime compilation, Fantomas, native helper Ruff lint/format, and diff checks pass.

[github-actions]

Python Type Checking Results (Pyright)

Metric	Value
Total errors	34
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_hash_set.py	18	Excluded
temp/tests/Python/test_applicative.py	12	Excluded
temp/tests/Python/test_nested_and_recursive_pattern.py	2	Excluded
temp/tests/Python/fable_modules/thoth_json_python/encode.py	2	Excluded