Emit deprecated tag for `deprecated` LSP diagnostics

[yangdanny97]

Describe the Bug

For the deprecated error code, we should emit LSP diagnostics w/ the deprecated tag: https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#diagnosticTag

That way, the editor can render a strikethrough over the function name. We should be careful what position we emit the diagnostic in. I think the strikethrough should only cover the function name, not the arguments.

Sandbox Link

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(Only applicable for extension issues) IDE Information

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[majiayu000]

Hi, I've been looking into this and traced the root cause.

Proposed approach: 1. Comment on issue per CONTRIBUTING.md rules. 2. Locate LSP diagnostic emission in pyrefly/lib/lsp where deprecated errors are published. 3. When error code == deprecated, add DiagnosticTag::Deprecated (value 2 per LSP spec) to the diagnostic's tags field. 4. Ensure the diagnostic range covers only the function name span, not arguments. 5. Run cargo test to verify. Prior PRs #1093/#1623/#1694 provide reference for how deprecated detection is structured.

I'll open a draft PR shortly. Happy to adjust based on your preference.