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 80.85106% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.71%. Comparing base (65909a2) to head (92ef9a2).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
.../parser/FlatCollectionSubDocAddOperatorParser.java	80.00%	6 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #272      +/-   ##
============================================
+ Coverage     80.68%   80.71%   +0.02%     
- Complexity     1396     1398       +2     
============================================
  Files           240      241       +1     
  Lines          6882     6928      +46     
  Branches        638      642       +4     
============================================
+ Hits           5553     5592      +39     
- Misses          914      919       +5     
- Partials        415      417       +2     

Flag	Coverage Δ
integration	80.71% <80.85%> (+0.02%)	⬆️
unit	52.88% <10.63%> (-0.30%)	⬇️

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[puneet-traceable]

do we do something for these operations to be idempotent?

[suddendust]

So, for this method that'd depend on the filter query containing a CAS clause, which the client has to pass. Something like lastUpdatedTime. document-store by itself doesn't care about idempotency.