docs(ai): Document enum values in top-level JSDoc comments

[dlarocque]

This fixes an issue with the documentation on DevSite, where enum values' JSDoc strings aren't extracted by our API documenter tool. For example, the ImagenPersonFilterLevel docs don't contain docs for each enum value, despite those explanations being in the source code here.

I removed all documentation from the type union declarations (type x = (typeof x..), because 1) they're not used by API documenter so they aren't in devsite, 2) VSCode type hints inherit the docs of the type union from the object:

(here we're hovering the definition of a HarmCategory type union, and we see the docs for the HarmCategory object)

[changeset-bot]

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Latest commit: 72961ec

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[google-oss-bot]

Size Report ¹

Affected Products

No changes between base commit (ccbf7ba) and merge commit (8057046).

Test Logs

• Base (ccbf7ba): https://github.com/firebase/firebase-js-sdk/actions/runs/18107463193
• Merge (8057046): https://github.com/firebase/firebase-js-sdk/actions/runs/18289884349

1. https://storage.googleapis.com/firebase-sdk-metric-reports/mEIuuAzviV.html

[google-oss-bot]

Size Analysis Report ¹

Affected Products

No changes between base commit (ccbf7ba) and merge commit (8057046).

Test Logs

• Base (ccbf7ba): https://github.com/firebase/firebase-js-sdk/actions/runs/18107463193
• Merge (8057046): https://github.com/firebase/firebase-js-sdk/actions/runs/18289884349

1. https://storage.googleapis.com/firebase-sdk-metric-reports/JiA60GAH9K.html

[hsubox76]

Unless I'm misunderstanding, removing the comments above the type unions actually does remove strings from devsite. There's two sections in devsite docs, Variables and Type Aliases, and the consts go under Variables and the string unions go under Type Aliases. That's not great but not really changeable without messing with api-documenter. They both have the same anchor name so if you click on one in the table of contents, you'll only go to the first one (under Variables), but the other one also exists and people could run into it by scrolling, or the short description in the table of contents.