feat(cubesql): Support more directions for FETCH statement

[ovr]

Previously, only FETCH <count> was supported. This adds handling for:

• FETCH NEXT (returns 1 row)
• FETCH FORWARD [count] (returns count rows, or 1 if omitted)
• FETCH ALL / FETCH FORWARD ALL (returns all remaining rows)

refs #10320

[codecov]

Codecov Report

❌ Patch coverage is 41.61074% with 87 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.44%. Comparing base (85d9e79) to head (103add9).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
rust/cubesql/cubesql/src/sql/postgres/error.rs	44.33%	59 Missing ⚠️
...st/cubesql/cubesql/src/sql/postgres/ast_helpers.rs	28.57%	25 Missing ⚠️
rust/cubesql/cubesql/src/sql/postgres/shim.rs	62.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10377      +/-   ##
==========================================
- Coverage   83.29%   78.44%   -4.85%     
==========================================
  Files         248      471     +223     
  Lines       74486    91801   +17315     
  Branches        0     3539    +3539     
==========================================
+ Hits        62041    72012    +9971     
- Misses      12445    19252    +6807     
- Partials        0      537     +537     

Flag	Coverage Δ
cube-backend	57.60% <ø> (?)
cubesql	83.28% <41.61%> (-0.02%)	⬇️

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