Explain why initState() is too early to depend on an inherited widget - #192727

When dependOnInheritedWidgetOfExactType() is called before initState() has completed, the error explains the rule like this:

When an inherited widget changes, for example if the value of Theme.of() changes, its dependent widgets are rebuilt. If the dependent widget's reference to the inherited widget is in a constructor or an initState() method, then the rebuilt dependent widget will not reflect the changes in the inherited widget.

That last sentence describes something that does not happen. The dependency is registered either way, so the dependent widget is rebuilt and its build() method does see the new value. What is never updated is whatever initState() read, because the framework calls initState() once per State object and does not call it again for those rebuilds. Reading the error as written, it is easy to conclude that depending on an inherited widget in initState() breaks the rebuild itself, which is what the issue reports as confusing.

This replaces that sentence with the actual reason, and adds the same reason to State.initState's documentation, which told readers not to call dependOnInheritedWidgetOfExactType() there but not why. The ErrorHint that points at build() and didChangeDependencies() is unchanged.

After:

When an inherited widget changes, for example if the value of Theme.of() changes, its dependent widgets are rebuilt. The framework calls initState() only once per State object and does not call it again for those rebuilds, so a value that initState() reads from an inherited widget is never updated when that inherited widget changes.

The existing test for this error (initState() dependency on Inherited asserts) only checks that something was thrown, so the message itself was untested; the new test covers it. ExpectFail now forwards the caught error so a test can inspect it.

The issue also suggests splitting _StateLifecycle.created into two states so the message can distinguish "in the constructor" from "inside initState()". I left that out: it is a debug-only bookkeeping change and the summary line already names the method it happened before.

Fixes #105705

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