Fix list concatenation ignoring outer list type context

[Ria-K912]

Fixes #21710

List literals already use an outer list[...] type context for bidirectional inference, so this works:

x: list[int | None] = [0, 1, 2, 10]

But concatenation did not forward that context, so both operands were inferred as list[int] and assignment failed due to invariance:

x: list[int | None] = [0, 1, 2] + [10]  # error before this PR

This PR special-cases + when the active type context is builtins.list, similar to the existing [...] * n handling: both operands are checked under that context before list.__add__ is applied. That covers list literals, list comprehensions, empty lists, and chained +.

Added testListAddInContext. Invalid cases still error, e.g. y: list[int] = [0] + ["x"].

[Ria-K912]

Re #21755 point-by-point:

Impact claims

1. “Patch will not parse cleanly” - False. No conflict markers; CI is green.
2. “Unresolved merge conflict markers” - False. Diff has none (<<<<<<< / ======= / >>>>>>>).
3. “x = a + [10] doesn’t exercise the bug” - Incomplete. That line is a secondary case; the List constant expression treated differently to list literal #21710 bug is covered by x = [0, 1, 2] + [10] and the comprehension case.
4. “P3 nested lists has no coverage” - Chained + is tested ([1] + [2] + [3]). Nested list[list[T]] was never claimed for this fix.
5. “P1/P2 depend on mangled check_op_expr” -False. No mangled code; change is a clean check_list_add when context is builtins.list.
6. “Failing-first violated” -Tests cover the intended behavior, including a negative case (y = [0] + ['x']).
7. “Verify the test exercises the bug” - Already done via the literal/comprehension cases above.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

prefect (https://github.com/PrefectHQ/prefect)
- src/prefect/utilities/callables/__init__.py:717: error: Incompatible types in assignment (expression has type "list[None]", variable has type "list[expr | None]")  [assignment]
- src/prefect/utilities/callables/__init__.py:717: note: "list" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
- src/prefect/utilities/callables/__init__.py:717: note: Consider using "Sequence" instead, which is covariant
- src/prefect/utilities/callables/__init__.py:719: error: Unsupported operand types for + ("list[None]" and "list[expr | None]")  [operator]

pydantic (https://github.com/pydantic/pydantic)
- pydantic/aliases.py:28: error: Incompatible types in assignment (expression has type "list[str]", variable has type "list[int | str]")  [assignment]
- pydantic/aliases.py:28: note: "list" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
- pydantic/aliases.py:28: note: Consider using "Sequence" instead, which is covariant
- pydantic/aliases.py:28: error: Argument 1 to "list" has incompatible type "tuple[str | int, ...]"; expected "Iterable[str]"  [arg-type]

dedupe (https://github.com/dedupeio/dedupe)
+ dedupe/labeler.py:451: error: Unused "type: ignore" comment  [unused-ignore]
+ dedupe/labeler.py:487: error: Unused "type: ignore" comment  [unused-ignore]

jax (https://github.com/google/jax)
- jax/_src/numpy/lax_numpy.py:3148: error: Incompatible types in assignment (expression has type "list[int]", variable has type "list[float64]")  [assignment]
+ jax/_src/numpy/lax_numpy.py:3148: error: List item 0 has incompatible type "int"; expected "float64"  [list-item]

[Ria-K912]

@hauntsaninja, @JelleZijlstra Could you please take a look at this PR? It fixes #21710 by forwarding outer list[...] context through list + (only for list constructors).