Tracking Issue: Fix Sqlsmith workarounds / refinements / bugs

[kwannoel]

Several features are currently not fully supported, hence some parts of sqlsmith are disabled, or use workarounds. We should re-enable them when their dependent issues are fixed.

Bugs:

• [x] #4220
• [x] #4344
• [x] #4434
• [x] #4508
• [x] #3908
• [x] https://github.com/risingwavelabs/risingwave/issues/6350

Workarounds:

• [x] #4057
• [x] #3935
• [x] #3939 workaround: #3929
• [x] #3900 workaround: #3929
• [x] #4058
• [x] #3908 workaround: #3956
• [x] #4737
  • [x] #4220
• [x] #4024
• [x] #7468
• [x] #6592

Refinements - Query Generator:

• [x] Look into gen_agg_expr, simplify if possible.
• [x] Clean up docstrings for various functions.
• [x] #4056
• [x] #4068
• [x] #4059
• [x] #4002
• [x] #4065
• [x] #4086
• [x] #4002
• [x] #3964
• [x] #4296
• [x] #7311
• [x] #4295
• [x] Generate more complex Time Window Functions: e.g. Different window slide.
• [x] #7467
• [x] #6945
• [x] Generate non-positive intervals for non Time Window Functions.
• [ ] https://github.com/risingwavelabs/risingwave/issues/6970
• [x] https://github.com/risingwavelabs/risingwave/issues/3844
• [ ] #8173
• [x] Generate non equi join.

Investigate:

• [x] Check for any feature in https://github.com/risingwavelabs/risingwave/issues/112 that haven't been tested on either batch mode or streaming mode
• [x] #7469

Refinements - Test suite:

• [x] #6796
• [x] Sqlsmith test: test skipped queries below threshold
• [x] #7901
• [ ] #8284 (high priority features)

Related: #2571

[kwannoel]

Some updates on what I intend to do:

• In order to keep progress of sqlsmith moving, I may implement some features, but disable in PR if they are blocked by some bugs.
• Then, most of the work that will be left is to enable / disable the implemented sqlsmith features as bugs get fixed.
• Once important features all tested, we can close this issue, consider sqlsmith work complete, move on to maintenance.
• Probably won't be able to cover all sql features, so for lower priority ones, just ignore first.

---

1. Enable features is higher priority (we want to ensure high coverage) and simpler, use this to track: https://github.com/risingwavelabs/risingwave/issues/7328.

2. We can have a separate issue to track lower priority work: https://github.com/risingwavelabs/risingwave/issues/7329 a. nice-to-haves e.g. test suite usability refinements. b. Low priority sql features to test. c. We can mark these as good first issue, so other developers / community can work on it :)

3. Long running features #7934 .