Implement ADD Subdoc Update for FlatPostgresCollection

[suddendust]

Add ADD sub-document update operator for flat collections

Summary

Implements the ADD operator for FlatPostgresCollection, enabling atomic numeric increment/decrement operations on both top-level columns and nested JSONB fields.

Changes

• Added [FlatCollectionSubDocAddOperatorParser] to handle ADD operations
• Registered the parser in FlatPostgresCollection.SUB_DOC_UPDATE_PARSERS

SQL Generated

Scenario	Generated SQL
Top-level column	"price" = COALESCE("price", 0) + ?::integer
Nested JSONB	"metrics" = jsonb_set(COALESCE("metrics", '{}'), '{sales,total}'::text[], (COALESCE("metrics"#>>'{sales,total}', '0')::integer + ?::integer)::text::jsonb, true)

Features

• Type-aware: Uses column schema type for top-level, infers from Java value type for JSONB. This information is then use for the apt type-cast.
• NULL-safe: COALESCE handles NULL columns/fields (treats as 0)
• Creates missing JSONB keys via jsonb_set(..., true)

[codecov]

Codecov Report

❌ Patch coverage is 70.21277% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.47%. Comparing base (6a2b05d) to head (469ead1).

Files with missing lines	Patch %	Lines
.../parser/FlatCollectionSubDocAddOperatorParser.java	68.88%	11 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #272      +/-   ##
============================================
- Coverage     80.52%   80.47%   -0.05%     
- Complexity     1396     1398       +2     
============================================
  Files           239      240       +1     
  Lines          6695     6741      +46     
  Branches        615      619       +4     
============================================
+ Hits           5391     5425      +34     
- Misses          900      910      +10     
- Partials        404      406       +2     

Flag	Coverage Δ
integration	80.47% <70.21%> (-0.05%)	⬇️
unit	54.35% <10.63%> (-0.32%)	⬇️

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