Fix readField during merge of objects sandwiched between circular objects.

[phryneas]

See #9315 (comment) for the user reproduction.

The test for this uncovers additional problems that might be fixed later.

The general problem here is a situation where an incoming object has another normalized object both as a parent and child.

Parent:1.foo -> Child:2.bar -> Parent:1

So, in this case, the merge function for Parent runs before the merge for Child has a chance to run - and the readField function that would like to access properties of Child cannot read them from the store yet.

This doesn't happen for non-circular objects, since these are merged from incomingById in a leaf-to-root order, but in this circular case, leaf and root are the same object - so no matter which order they are merged in, a child will have unmerged fields.

I'm not happy with this solution, so putting it as draft for now. I'd be happy to feedback.

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commit: 1ee5b01

[phryneas]

Kinda unrelated, but it came up during the PR. getFieldValue<string> is a hidden cast to string, even though the function can always return undefined, too.

[apollo-librarian]

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* (developer-tools)/react/(latest)/caching/advanced-topics.mdx
* (developer-tools)/react/(latest)/caching/cache-field-behavior.mdx
* (developer-tools)/react/(latest)/data/fragments.mdx
* (developer-tools)/react/(latest)/data/mutations.mdx

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