Adds document_symbol support for pyrefly

[jackd]

Adds document_symbol support for pyrefly

I originally wrote this as an issue, but since I had the fix I thought I might as well offer it. I want to be completely up front and admit I'm not a rust person, this was agent-authored, and I'm not going to pretend to understand it. I hope that doesn't offend anybody, and I similarly won't be offended if this PR is promptly rejected on that basis.

I could have added tests, but given I wouldn't understand them either I don't see the value of including that to that project.

What I can say is the document_symbol didn't work with pyrefly before the change, and it did afterwards.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)

Checklist

• I have read the CONTRIBUTING guide
• My code follows the project's coding style
• I have added tests that prove my fix/feature works
• All new and existing tests pass
• I have updated the documentation accordingly
• I have updated the CHANGELOG.md (for user-facing changes)

Additional Notes

Config that works with it:

[[workspace.language_extensions]]
extensions = ["py", "pyw", "pyi"]
language_id = "python"

[[lsp_servers]]
language_id = "python"
command = "pyrefly"
args = ["lsp"]
file_patterns = ["**/*.py"]

[Copilot]

Pull request overview

This PR updates the LSP client initialization handshake to advertise textDocument/documentSymbol support, improving compatibility with servers like pyrefly that may gate symbol responses on declared client capabilities.

Changes:

• Add text_document.document_symbol client capabilities during LSP initialize.
• Enable hierarchical document symbol support (hierarchical_document_symbol_support: true).

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[Copilot]

This change adds document_symbol to the client capabilities, but there’s no test asserting that the initialize request includes this capability. Since this module already has unit tests, consider adding a small unit test (or factoring out capability construction) to prevent regressions where document_symbol support is accidentally dropped again.