Fix PostgreSQL function argument default parsing

[Dan-Knott]

Problem

When parsing PostgreSQL function defaults, pg_proc.proargdefaults converted with pg_get_expr() returns a single comma-separated string containing all default values, rather than an array.

This creates several challenges:

1. The default values need to be split from the comma-separated string
2. These values must then be aligned to the rightmost input arguments (since only trailing arguments can have defaults in PostgreSQL)
3. Edge cases exist where commas might legitimately appear within default values themselves, making naive string splitting unreliable

Solution

Instead of parsing and realigning the comma-separated string, this PR uses PostgreSQL's pg_get_function_arg_default() function to retrieve the default value for each argument individually. This function:

• Takes an argument position and returns the default for that specific argument
• Returns the properly formatted default expression
• Avoids any ambiguity around parsing comma-separated values
• Eliminates the need for complex right-alignment logic

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.80%. Comparing base (d8ab535) to head (a389397).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #132   +/-   ##
=======================================
  Coverage   89.79%   89.80%           
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  Files          86       86           
  Lines        4096     4099    +3     
  Branches      850      850           
=======================================
+ Hits         3678     3681    +3     
  Misses        338      338           
  Partials       80       80           

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[DanCardin]

looks great!