Remove Belt xxxUndefined API

[cknitt]

Summary

This PR removes the legacy Belt APIs that return Js.undefined and standardizes callers on the existing option-returning and OrThrow APIs instead.

It updates the public Belt runtime interfaces, removes the corresponding generated CJS/ESM exports, and refreshes affected tests and fixtures.

Motivation

• This prepares the broader effort to get rid of the Js namespace altogether
• undefined<'a> is likely a leftover from the early days before option<'a> compiled to undefined for None
• These days, option<'a> should be used instead of undefined<'a>

Changes

• Remove xxxUndefined helpers from Belt Array, Map, Set, MutableMap, MutableSet, MutableQueue, and MutableStack APIs
• Remove the corresponding internal AVL tree/set helpers that only existed to support those public APIs
• Add/use the OrThrow variant where needed (Belt.MutableStack.popOrThrow)
• Update generated runtime artifacts under packages/@rescript/runtime/lib
• Update affected runtime tests and fixture code to use option-returning replacements

Migration

Use the existing non-undefined APIs instead:

• getUndefined -> get
• minUndefined / maxUndefined -> minimum / maximum
• minKeyUndefined / maxKeyUndefined -> minKey / maxKey
• peekUndefined / popUndefined -> peek / pop
• topUndefined / popUndefined on Belt.MutableStack -> top / pop
• use *OrThrow when throwing behavior is desired

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commit: ea0d555