Include instance attributes in workspace symbol search

[dantrapp]

Summary

Workspace symbol search misses attributes initialized in method bodies, including self._private_member = 1 from #4688. The existing search already finds _private_method. This change adds receiver-attribute assignments to the cached flat symbol table, returning the attribute's name location with its enclosing class as the container.

The collector handles ordinary and annotated assignments, chained assignments, and unpacking. It excludes function locals, unrelated object targets, and syntactically marked static methods, and resets receiver tracking at nested function/class boundaries. Auto-imports still use the unchanged export table.

The collector remains syntax-based: it recognizes @staticmethod and qualified .staticmethod decorators, but does not resolve decorator aliases or track receiver rebinding.

Fixes #4688.

Test Plan

• First commit (9153364) reproduces the failure: the method control passes, while the instance-attribute test returns zero matches and fails.
• cargo test -p pyrefly --lib workspace_symbol: 16 passed, including workspace-symbol protocol tests and lightweight tests for both Require::Everything and Require::Indexing.
• cargo test -p pyrefly --lib export::: 44 passed, including assignment forms, receiver scope, and static-method exclusions.
• cargo test -p pyrefly --lib test::lsp::completion::: 142 passed.
• python3 test.py --no-test --no-tensor-shapes --no-conformance --no-jsonschema: passed.
• cargo build -p pyrefly: passed; the executable reports pyrefly 1.3.0-dev.4.

Local validation used Rust 1.96.0. Unrelated formatter changes to existing imports in test/pydantic/util.rs and test/suppression.rs were excluded from this PR. The full cross-platform suite is left to CI.

AI assistance: OpenAI Codex performed the investigation, implementation, and test execution.

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@kinto0 has imported this pull request. If you are a Meta employee, you can view this in D119740446. (Because this pull request was imported automatically, there will not be any future comments.)