[ty] Fix structural pattern binding inference (#26411)
## Summary
Follow-up to #25941 and prerequisite for #25942.
The earlier PR extended successful-pattern binding inference to class
and mapping patterns. This PR fixes edge cases in that analysis before
the same structural machinery is reused for subject narrowing.
```python
from typing import Literal
class IntPayload:
tag: Literal["int"]
payload: int
class StrPayload:
tag: Literal["str"]
payload: str
def handle(
value: object,
Pattern: type[IntPayload] | type[StrPayload],
) -> None:
match value:
case Pattern(tag="int", payload=item):
reveal_type(item) # int
```
Union-valued class expressions are now analyzed one class at a time, so
a discriminating child pattern also selects the corresponding member
types. Generic class patterns preserve specializations known from the
subject, while unrelated non-final classes retain member types that can
overlap through multiple inheritance. Runtime class matching uses an
unknown specialization rather than a type-parameter default.
Mapping entries are extracted after the successful pattern has
established the mapping interface, which keeps fully static `object`
subjects from introducing `Unknown`. Later `or`-pattern alternatives use
mutation-safe fallthrough types, so a failed sequence alternative does
not leave stale shape constraints on a value captured by another
alternative.
