fix(python): make the assert_not_equal failure message describe the failure

[dbrattli]

Follow-up to #4776, where the same wart surfaced on the Beam target.

On a failing Assert.NotEqual, assert_not_equal renders:

Expected: 1 - Actual: 1

The two values being equal is precisely why the assertion failed, so the message describes a passing assertion. Staring at a red test, it tells you nothing — and it briefly reads as though the test framework is broken. It happens because assert_equal and assert_not_equal share one format string:

def assert_not_equal[T](actual: T, expected: T, msg: str | None = None) -> None:
    if equals(actual, expected):
        raise Exception(msg or f"Expected: {expected} - Actual: {actual}")   # same string as assert_equal

Now it says what was actually expected — some value other than this one:

Expected not equal to: 1 - Actual: 1

Prior art

xUnit, which these same tests run against on .NET before being transpiled, goes out of its way to distinguish the two cases:

Assert.Equal() Failure: Values differ          Assert.NotEqual() Failure: Values are equal
Expected: 1                                    Expected: Not 1
Actual:   2                                    Actual:       1

The Beam target already words it this way (#4775). This brings Python into line, keeping the Fable house format (Expected... - Actual...) rather than adopting xUnit's layout.

Scope: JS is deliberately left alone

fable-library-ts's Util.ts:assertNotEqual has the identical wart. I have not touched it — that's a call for the JS/TS maintainers, and I'd rather use this PR to ask than to unilaterally change a message on the most-used target.

@fable-compiler maintainers: do you want the same one-line change on JS/TS (and Dart/Rust, which don't currently implement Assert at all), so all targets agree? Happy to fold it in here or leave it for a separate PR — or to drop this one if you'd rather all targets keep matching each other, warts and all.

Compatibility

fable-library-py is on PyPI and must stay backward-compatible within a minor version. This changes no signature, no raising behaviour, and no return value — only the human-readable text of a message that, by construction, is produced only on an already-failing assertion. Nothing in the repo depends on the old wording. Flagging it explicitly since AGENTS.md asks for care with the semantics of existing fable-library-py functions.

Tests

New src/fable-library-py/tests/test_assert.py covers both functions: pass, fail, and custom-message paths. The not-equal message test fails on main and passes here; the rest pass either way.

Full fable-library-py suite: 444 passed. Pyright clean on both files.

🤖 Generated with Claude Code

[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