Fix completion docstrings for stubbed attributes

[paranoa233]

Summary

• use the executable .py definition when loading documentation for attribute completions
• keep completion types and signatures sourced from the .pyi stub
• add an LSP regression test covering paired .py and .pyi modules

Fixes #4057.

Why

Attribute completion documentation used the default stub-first definition preference. When a stub supplied the type but omitted a docstring, completion stopped at the .pyi definition instead of using the executable-module fallback that hover already uses.

Selecting the executable definition for this documentation-only lookup reuses that existing fallback without changing the stub-derived completion type.

Test plan

• cargo +stable-x86_64-pc-windows-gnu test -p pyrefly --test pyrefly_lsp_interaction_tests completion_attribute_prefers_py_docstring_over_pyi --no-fail-fast
• cargo +stable-x86_64-pc-windows-gnu test -p pyrefly --lib docstring --no-fail-fast
• cargo +stable-x86_64-pc-windows-gnu test -p pyrefly --test pyrefly_lsp_interaction_tests hover_attribute_prefers_py_docstring_over_pyi --no-fail-fast
• py -3.11 test.py --no-test --no-tensor-shapes --no-conformance --no-jsonschema

AI disclosure

This pull request was prepared and submitted by an AI agent. The change was validated with the repository's targeted tests, formatting, and linting workflow.

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[meta-codesync]

@grievejia has imported this pull request. If you are a Meta employee, you can view this in D111468403.

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