Clean up TODO comments with missing documentation links

[abrookins]

Problem

The core model code contains 34 TODO comments. While many are non-blocking commentary, several indicate incomplete implementations or missing documentation:

Documentation TODOs (user-facing)

• Line 1541-1545: raise QuerySyntaxError("... See docs: TODO")
• Line 3118, 3123: "... See docs: TODO"

Implementation TODOs

• Line 1412: # TODO: Implement NOT_IN, test this...
• Line 1759-1760: # TODO: Response type? # TODO: Better error detection for transactions.
• Line 1765: # TODO: Better response type, error detection

Solution

1. Remove or address documentation TODOs with actual doc links
2. Either implement missing features or document limitations
3. Clean up commentary TODOs that are no longer relevant

Impact

• Incomplete error messages confuse users
• Missing implementations may cause unexpected behavior
• Code cleanliness for 1.0 release