[tailwindcss-vite] Add Register dependency messages logline (#19611)

The PostCSS implementation has a debug logline for registering
dependencies but this is missing for @tailwindcss/vite.

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## Summary

Adds the "Register dependency messages" instrumentation span to the Vite
plugin's dependency registration code, matching the existing
implementation in @tailwindcss/postcss.

This ensures consistent debug instrumentation across both bundler
integrations when DEBUG is enabled.

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## Test plan

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