[match-case]: mapping pattern narrowing inconsistent with equivalent `isinstance` narrowing

Code sample in [pyright playground](https://pyright-play.net/?code=GYJw9gtgBALgngBwJYDsDmUkQWEMoCGAzkQKZ4D68CpAUKJFAMZgA2rpTMSYKRAdAQBGTTNlz4AsgQTJ0tWgBNSwWKSIwKqDQRRNSACgAeALiiKkXKAB9MKGAEooAWgB8UAHK9SJ2lH%2BYqkhE2jC6%2BsYANFDSsqhoDr4ByYQk5JrUhkbRFlxOUADEUICg5IDwf34BpKxkSSn%2BxGSUmVF2jv5FZQrKqjDqmhAEMEwAFsZmufi2qG1unt61-gNDw1CmFXVMxKRQAN4AvmaFUKQAjgCuSABuBBz2sGCYIXxhelnRsXIJ63UBDelUiDe5ksbSOgBlyco-ZhbKAUBZQ1KNDKAlrTfJFCFAA)

```python
from typing import assert_type
from collections.abc import Mapping

def test_instance(x: dict | int) -> None:
if isinstance(x, Mapping):
assert_type(x, dict) # ✅️
else:
assert_type(x, int) # ✅️

def test_match(x: dict | int) -> None:
match x:
case {}: # equivalent to isinstance(x, Mapping)
assert_type(x, dict) # ❌️
case _:
assert_type(x, int) # ❌️
```

Additionally, there are some failures related to `TypeDict` with mapping patterns: Code sample in [pyright playground](https://pyright-play.net/?code=GYJw9gtgBALgngBwJYDsDmUkQWEMoAqiApgCYAiSAxjADRQCGAzk8XgPrwLEBQokUKmAA2w4jSRgUTAHQMARlUzZc%2BALIMEydDx6liwWMSYx2qEwxRViACgAeALiilq%2BAD6YUMAJRQAtAB8UAByUsQOPFBRmIZITOYwltb29BpaqGjeEdE5jCxsply2dvQuNN6R0cTCrNm5UcysHEUpnj66%2BoYwxqYQDDBUABb2TmXubb6BIWF1UX0Dg1COlfVUzMRQAN4Avk5QAMRQxACOAK5IAG4MYl6wYJjx0olWxama2pkr9dGNBZwkrTGFW%2BgnWUHYsxBeSahQBJQmuh4VGEjSgAEEbERuBRXFkoAhGh0DEYTGYnkliOwMY50VAPKgfP4gqEUOEvkhYo8LC9WmkPlkvjlfs04fQ0cCqjU2SDhbDuK0GRU9MTuqT5kMqSNafSvJNmTMvurFssQWtWFtdlFDidzlcbvgYPc4gkKbz3hkJVDoX8WvDxYLomaNhCA-VZf95fDFVaoIAZckA8H-ekXyhl09H0aPABhIMSkKAAbVlfiKAF1dMjUQAhTEkHHlSEoBgQcJQEwgIldHpk7nWdjVmmVtPRqYs6XRDkPF08%2BF8j2QoX5ZOvKCVz1HKXzn6LuXLxUdkm9foa-tOQc6xkjg05I1LTdRINbABEjebj6c7EtByOZ0u12It0dKAbGdclpzedJ0F8Sw82fJtiEfTwljXGVtwjZdV1DUFzRDL0GlQ30M11RFOgPdgjXYO1TkpIo%2By1M8JiZaZWUhG8TW%2BB9Nlg18nDbGxvF2TCcmtH87X-B17mArlnmSGd3UgxgUDzECe2KGQ0GIGAbC4%2BDvHoNtvGQ75wwIldDPvMEcNw4zRQRIA)

Reproduce both cases from the linked Pyright Playground samples, comparing test_instance with test_match and checking the TypeDict mapping-pattern failures. Start with the assert_type results and trace the narrowing behavior in Pyright’s relevant tests and implementation. Done means equivalent Mapping and match narrowing agree, with the reported TypeDict cases covered.

Written by the indexing model from the issue text.