In order to prevent bad titles from entering the database,
PageIdentityValue should not allow instances that have obviously broken
dbeys, just like it does not allow construction for a special page.
Note however that for now, we allow PageReferenceValue to represent
thigns that are not actual pages, such as titles like "User:#1234" which
are used in the block log to represent autoblock targets. Similarly,
a PageReferenceValue can represent a special page.
This patch introduces a convenience method for constructing
PageIdentityValues in a context where it is uncertain whetehr the title
is valid, providing an opportunity to the caller to report in which
context the bad values were found.
In addition, this patch adds some documentation and testing, to clarify
the semantics of methods in PageStore with respect to invalid and non-
proper titles.
Bug: T282070
Change-Id: I8a44d9e73dda8fe38d009847332946809767d0fa
62176bec1b