Fix: ensure unpacked **kwargs have string-compatible keys (#20706)

[Vikash-Kumar-23]

Summary
This PR addresses issue #20706, where mypy failed to catch a TypeError occurring at runtime when non-string keys were used within dictionary unpacking (**).

In Python, keyword unpacking via the ** operator requires all keys to be valid identifiers (strings). While mypy caught this in most contexts, it missed the error inside dict() constructors because the unpacked mapping was incorrectly being matched to positional parameters in the constructor's overloads.

Changes
Updated visit_dict_expr: Modified this method in mypy/checkexpr.py to validate **expr items before they are unpacked.

Key Validation: Integrated a check to ensure unpacked expressions are mappings with keys compatible with builtins.str.

Error Diagnostic: Triggered invalid_keyword_var_arg when non-string keys are detected to align static analysis with runtime behavior.

Checklist
[x] Read the Contributing Guidelines.

[x] Added tests for all changed behavior.

[x] Verified that my code passes the local test suite using pytest mypy/test/testcheck.py.

Verification
Verified locally with the following reproduction script:

Python
d: dict[int, int] = {10: 20}
dict(**d) # Now correctly reports: error: Argument after ** must have string keys [arg-type]

[hauntsaninja]

Add tests, analyse the primer diff, make sure you account for this #20706 (comment)

[A5rocks]

The interesting thing is that we already check this, I guess just not for the dict call:

def f(**kwargs: int) -> None:
    pass

f(**{1: 1})  # E: Keywords must be strings

So IMO we just need to find what's doing skipping that check and not do that.

[hauntsaninja]

Yes, see my note here: #20706 (comment)

[Vikash-Kumar-23]

I have refactored the fix based on the feedback from @hauntsaninja and @A5rocks.

I found that dict() calls are transformed into DictExpr during semantic analysis, which caused them to bypass standard keyword validation. I've introduced a from_dict_call flag to the DictExpr class to preserve this context. This allows checkexpr.py to utilize the existing is_valid_keyword_var_arg logic to catch non-string keys in dict(**kwargs) while avoiding false positives for standard dictionary literals.

I also added a permanent unit test in test-data/unit/check-expressions.test as requested.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[A5rocks]

Is there a reason we have to do this? Maybe this is from older mypy where a workaround was required... Can you test the fallout from removing this?

[A5rocks]

Nevermind, I see from blame it's required for TypedDict assignments. That doesn't seem completely necessary (surely we can special case TypedDict type context for dict calls?), but I guess that's more of a followup.

EDIT: however, I think it may be cleaner simply to return None for any dict w/ non-string keys at this point, rather than add an extra attribute