[pyrefly] Contextualize dict subscript literals for Any

[lyydsheep]

Fixes #4301

A dict literal assigned through an explicitly annotated dict[str, Any] subscript now receives a soft value hint, avoiding a spurious implicit-any-empty-container diagnostic. Unannotated and narrow dictionary targets retain their existing diagnostics and inference behavior.

Validation:

• cargo test -p pyrefly test::inference::test_implicit_any
• cargo test -p pyrefly test::dict
• python3 test.py --no-test --no-tensor-shapes --no-conformance --no-jsonschema

The local .scratch planning files are intentionally not part of this PR.

[meta-codesync]

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[IBlackVoid]

Heads up rather than having this show up unannounced — I opened #4409 with a more general take on the same bug. You got here first, and I'd rather flag it than surprise you.

Same idea, but keyed on whether the placeholder can escape rather than on the target's shape, which turned out to matter more than I expected. Measured, both patches applied to the same base and run on the same inputs:

case	main	#4391	#4409
dict[str, Any] (the report)	error	fixed	fixed
dict[int, Any]	error	error	fixed
MutableMapping[str, Any]	error	error	fixed
defaultdict[str, Any]	error	error	fixed
list[Any]	error	error	fixed
hand-written __setitem__	error	error	fixed
x = {} then x["y"] = {...}	error	error	error

The key-type condition in the qname match is what narrows yours: the key has nothing to do with whether the value is Any, so dict[int, Any] gets missed. Real code hits this — test_configparser.py does cf["A"] = {} where cf is a MutableMapping, not a builtins.dict.

Digging further also turned up a second root cause behind the same diagnostic: checking against Any never pins the placeholder either, which produces the identical false positive on attribute assignment, return position, default arguments and TypedDict fields.

Genuinely happy to fold this into your PR instead of landing it separately if you'd prefer — the analysis is the part I care about, not whose branch it goes in on.