fix(common): preserve literal key union in KeyValuePipe.transform() - #70200

Previously, when you passed an object typed like
Record<'a' | 'b', number> into the `keyvalue` pipe, TypeScript would
"forget" that the keys could only ever be 'a' or 'b', and just tell
you the key was a plain `string` instead. So code like this used to
fail to compile, even though it's correct:

```ts
  const input: Record<'a' | 'b', number> = {a: 1, b: 2};
  const result = pipe.transform(input);
  const key: 'a' | 'b' = result[0].key; // error: string is not 'a' | 'b'
```

This happened because the pipe has multiple overloaded versions of
transform(), and TypeScript checks them top to bottom, using the
first one that matches. The "number keys" overload was listed first,
and it happened to also match string-keyed objects by accident, so
it "won" before the correct "string keys" overload ever got a
chance to run.

The fix just reorders those two overloads so the string-keys one is
checked first. Nothing about runtime behavior changes — objects with
actual numeric keys (e.g. Record<1 | 2, string>) still correctly
report their keys as plain `string`, matching what Object.keys()
really returns at runtime.