INTPYTHON-855 Create unique indexes in a way that can be used by the query planner

[sophiayangDB]

INTPYTHON-855

Changes in this PR

Before this change, exact lookups were generated as plain equality predicates in lookups.py. Since those queries did not include the type conditions required by partial unique indexes, MongoDB often chose a collection scan instead of using the index. This patch adds the corresponding type filters to exact lookups so the planner can select the intended partial unique index.

Test Plan

Added unit tests in tests/lookup_

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is the intention of the code captured in relevant tests?
• If there are new TODOs, has a related JIRA ticket been created?

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Have you checked for spelling & grammar errors?
• Is all relevant documentation (README or docstring) updated?

[timgraham]

$exists matches if the field is null, which we don't want (that explains at least some of the test failures).

Consider this advice: "Null vs. Missing: $exists: true will return documents where a field is explicitly set to null. If you want documents where a field exists and is not null, combine it with $ne: null"

[timgraham]

Is this branch used? Offhand, I can't think of why db_type would be None. That would denote a virtual field that don't exist in the database, and such fields wouldn't be indexed... (perhaps this is AI nonsense.)

[timgraham]

Conceptually, it makes sense to me that the partialFilterExpression isn't needed if the field can't contain null values. My concern is that we would also have to make AlterField modify any/drop any relevant indexes when a field changes from null to not-null (or vice versa).