Fix theme-aware constant highlighting

[WilliamK112]

Summary

• remove the hard-coded Dark+ foreground color for readonly-variable semantic tokens
• retain a color-neutral bold rule so all-caps constants remain visually distinct across themes
• keep the existing variable.other.constant.python TextMate scope fallback

This lets the active VS Code theme choose the constant color while preserving the visual distinction added for #1780. Light Modern and Dark Modern were both verified in isolated Extension Development Host profiles; screenshots and details are included in the review thread.

Fixes #4473

Validation

• npm run package
• repository formatting/lint command from AGENTS.md
• JSON assertion that the readonly-variable rule is color-neutral and bold
• live Light Modern and Dark Modern extension-host checks against a locally built Pyrefly LSP
• git diff --check

AI disclosure

I used an AI coding assistant to help inspect the extension configuration, implement the change, and validate the result. I manually reviewed the final diff and visual evidence.

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D115356762. (Because this pull request was imported automatically, there will not be any future comments.)

[yangdanny97]

Review automatically exported from Phabricator review in Meta.

[yangdanny97]

@WilliamK112 can you confirm that #1780 is still fixed after your change? since this effectively reverts part of the original fix for that.

screenshots of light and dark themes would be useful evidence, the current test plan is insufficient for this change, since this is not covered by automated testing.

[yangdanny97]

see other comment

[WilliamK112]

Thanks for flagging #1780. The first live check showed that simply removing the override was not sufficient: Dark Modern still distinguished constants via the retained variable.other.constant.python scope, but Light Modern did not. I adjusted the PR so the extension now applies only bold: true to variable.readonly:python, without setting a foreground color.

That preserves #1780's visual distinction in themes that do not assign constants a separate color, while #4473 remains fixed because each active theme still controls the actual color.

Light Modern

Dark Modern

Both screenshots are from isolated VS Code Extension Development Host profiles running this PR's extension against a locally built Pyrefly LSP. The logs confirm textDocument/semanticTokens/full completed and the sample file had zero diagnostics.

Validation on head d1761ae48:

• npm run package
• repository formatting/lint command from AGENTS.md
• JSON assertion for the color-neutral bold rule
• git diff --check

[yangdanny97]

Thank you!