fix(js/ts): Fix JSX props with long string values causing compile error (fixes #3839)

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Closes #3839

Root Cause

When a JSX prop's string value exceeds ~100 characters, Fable optimises the Fable AST by hoisting the string into a Let binding. The resulting expression changes from:

Value(NewTuple([StringConst "className"; StringConst "aaaa...long..."], ...), ...)
```

to:

```
Let(x, StringConst "aaaa...long...", Value(NewTuple([StringConst "className"; IdentExpr x], ...), ...))

The transformJsxProps function in Fable2Babel.fs only matched the direct MaybeCasted(Value(NewTuple([key; value], ...), ...)) pattern. When the string was Let-wrapped, no arm matched and the fallthrough addError fired with "Cannot detect JSX prop key at compile time".

Fix

Added a new match arm in transformJsxProps that handles MaybeCasted(Let(_, letValue, Value(NewTuple([key; _], ...), ...))). In this pattern:

• letValue is the actual long string value (what was bound)
• key is the prop name, extracted from inside the Let body's tuple
• the _ inside the tuple is the IdentExpr reference to the Let-bound variable (not needed)

The fix correctly uses letValue as the prop value, exactly as if the string had been written inline.

Test

Added divWithLongClassName to the JSX integration test in Components.fs with a string of 98 'a' characters (above the ~100-char threshold), and the corresponding expected output to Components.jsx.expected.

Trade-offs

• Minimal change: only Fable2Babel.fs (the JS/TS code generator) is modified
• No other targets are affected
• The children prop with a long string value is also handled correctly in the new arm

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