[@astrojs/rss] Quality-of-Life Improvement to items property-related error

[cdvillard]

Changes

This commit addresses a quality-of-life concern when setting up a RSS feed using collections. Specifically:

• it provides more context to the error message thrown when the object passed to the items property is missing any of the three required keys or if one of those keys is mistyped.
• it adds a link to provide additional context and instruction

Testing

I tested this change similarly to how I encountered #9171. Using the examples/blog project, I simply messed around with the properties of items to ensure an error would render, and that my changes would build and reflect appropriately.

Before:

After:

Docs

This shouldn't necessitate any changes to the docs as this way entirely done to help improve the error solely. Usage of the package remains unchanged.

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[cdvillard]

Just to put it out there, I'm not married to the copy language used, nor the method. I was a little confused by the use of errorMap on line 105. I tried to define a new conditional there to catch invalid_union types, but despite building fine, it never seemed to catch it. I have no deep experience with Zod, so I'm sure it could be something specific to the rssOptionsValidator ZodObject alone.

[sarah11918]

A docs review only: the proposed error message is fantastic. Thank you for the care and attention to detail @cdvillard! Massaged the changeset a little, so see what you think about that.