fix: set null publishedDate for yanked versions

[mbani01]

This pull request refactors how NuGet package published dates are parsed and handled, centralizing the logic for filtering out sentinel "unlisted" dates and improving code clarity. The main changes are:

Date Parsing Improvements:

• Added a new parsePublishedDate function to handle NuGet's sentinel unlisted date (1900-01-01T00:00:00Z) and return null for absent or invalid dates.

Refactoring and Consistency:

• Replaced all direct Date parsing and filtering logic for published dates in normalizeNuGetPackage with calls to the new parsePublishedDate function, ensuring consistent handling of unlisted or invalid dates throughout the code.
• Updated the assignment of publishedAt in version objects to use parsePublishedDate, further centralizing date validation.

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Note

Low Risk
Small refactor in NuGet normalization with clearer date filtering; behavior change is limited to published metadata for unlisted versions.

Overview
Centralizes NuGet published date handling in a new parsePublishedDate helper that returns null for missing values, invalid dates, and NuGet’s unlisted sentinel (1900-01-01T00:00:00Z).

normalizeNuGetPackage now uses that helper for firstReleaseAt / latestReleaseAt aggregation and for each version’s publishedAt, so yanked/unlisted versions no longer surface a bogus 1900 publish timestamp.

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[Copilot]

Pull request overview

This PR refactors NuGet package normalization to centralize how “published” dates are parsed, ensuring NuGet’s unlisted/yanked sentinel publish timestamps don’t leak into normalized metadata.

Changes:

• Adds a parsePublishedDate helper that returns null for missing/invalid dates and NuGet’s sentinel “unlisted” date.
• Replaces inline Date parsing/filtering in normalizeNuGetPackage with the helper for firstReleaseAt, latestReleaseAt, and per-version publishedAt.

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