fix: Timestamp cell tooltip renders dark text on the dark tooltip background

[nishantmonu51]

Hovering a timestamp cell in a model or source preview showed the value as near-black text on the dark tooltip, so it could not be read. This affects Rill Developer as well as cloud editing.

• Timestamp.svelte baked text-fg-primary into its class list next to the color prop that the cell tooltip sets to text-fg-inverse. Tailwind emits the text-fg-* utilities alphabetically, so primary lands later in the stylesheet and wins. Only timestamp columns were affected because the other data-type components do not hardcode a color.
• The fix drops the hardcoded class. Base.svelte already defaults to text-fg-primary when no color is passed, so timestamps outside the tooltip render exactly as before, in both light and dark mode.
• Adds FormattedDataType.spec.ts, which asserts that a rendered value carries only the requested text-fg-* class. It fails on the old component.

Steps to reproduce:

1. Open any model or source whose preview has a timestamp column, locally or in cloud editing.
2. Hover a cell in that column.
3. The first line of the tooltip (the full value) is unreadable, while the "Copy this value to clipboard" line below it is fine.

Closes APP-950

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!