[TSP] Return contextual expected type from getExpectedType

[rchiodo]

Summary

typeServer/getExpectedType was returning the inferred (computed) type at the cursor instead of the contextually expected type that the position imposes. For example, in x: int | str = 4, querying on 4 returned int rather than int | str.

This change makes get_expected_type_at_position walk the covering AST nodes innermost-first and, when the cursor lies in a context that supplies an expected type, look up that contextual type. Recognized contexts:

• Call argument (positional or keyword) — uses the parameter type declared by the callee, via Transaction::expected_call_argument_type (now pub(crate) so the non-WASM server can call it).
• return <expr> — uses the enclosing function's return annotation (resolved via get_type_trace over the annotation's full range so unions like int | str are preserved instead of collapsing to the leading identifier).
• Parameter default def f(x: T = <expr>) — uses the parameter's annotation T.
• Annotated assignment RHS x: T = <expr> — uses the LHS declared type T.

Other contexts (yield expressions, container-literal elements under an annotation, etc.) still fall back to the inferred type at the cursor; tests document that behavior so future fixes can flip the expectations.

Fixes microsoft/pylance-release#8060

Test Plan

Added 8 new tests in pyrefly/lib/test/tsp/tsp_interaction/get_type_queries.rs covering each newly supported context plus the documented fallbacks:

• test_get_expected_type_call_positional_argument
• test_get_expected_type_call_keyword_argument
• test_get_expected_type_return_statement_value
• test_get_expected_type_default_parameter_value
• test_get_expected_type_yield_falls_back_to_value_type
• test_get_expected_type_list_element_falls_back_to_container_type
• test_get_expected_type_dict_value_falls_back_to_container_type

All 14 test_get_expected_type* tests pass:

cargo test -p pyrefly --lib test_get_expected_type
test result: ok. 14 passed; 0 failed; 0 ignored; 0 measured

Also ran python3 test.py --no-test --no-conformance --no-jsonschema — formatting and linting clean. cargo clippy --all-targets --all-features produces no new warnings (only 2 pre-existing unused-import warnings in pyrefly_lsp_interaction_tests, unrelated to this branch).

[github-actions]

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

[kinto0]

nit: we probably don't need an integration test for these. we can probably just use a normal unit test

[meta-codesync]

@kinto0 has imported this pull request. If you are a Meta employee, you can view this in D107524019.

[kinto0]

discussed offline. closing