Subdoc SET Operator for Flat Collections

[suddendust]

PR Summary: Implement SET Operator for Flat PostgreSQL Collections

What

Adds support for the SET update operator on flat PostgreSQL collections, enabling sub-document updates on both top-level columns and nested JSONB paths.

Why

To provide MongoDB-like $set semantics for flat collections, allowing updates to specific fields without replacing entire documents.

Behavior

SET operator handles these scenarios:

Scenario	Behavior
Field not in schema	Skipped (with SKIP strategy) or throws (with THROW strategy)
Field is in schema but is NULL	That value is SET
JSONB column is NULL	Initializes to {} and sets the key
JSONB path exists	Updates the value
JSONB path doesn't exist	Creates the key

Key behaviors:

• For JSONB columns, uses jsonb_set(..., true) to create missing keys
• Uses COALESCE(column, '{}') to handle NULL JSONB columns
• When all update paths are skipped (e.g., none resolve to valid columns), the update is a no-op rather than failing

Assumptions

1. Depth-1 JSONB paths only — Nested paths within JSONB columns are assumed to be at most 1 level deep (e.g., customAttributes.myKey). Deeper paths would require recursive path creation which is not implemented yet.

2. MissingColumnStrategy applies to updates — The existing missingColumnStrategy configuration (SKIP/THROW/IGNORE_DOCUMENT) governs behavior when update paths don't resolve to schema columns.

[codecov]

Codecov Report

❌ Patch coverage is 84.66667% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.70%. Comparing base (a45e01d) to head (d0bc0ab).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...documentstore/postgres/FlatPostgresCollection.java	88.49%	7 Missing and 6 partials ⚠️
.../parser/FlatCollectionSubDocSetOperatorParser.java	78.57%	6 Missing ⚠️
...cumentstore/postgres/update/FlatUpdateContext.java	55.55%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #267      +/-   ##
============================================
+ Coverage     80.52%   80.70%   +0.17%     
- Complexity     1392     1394       +2     
============================================
  Files           237      239       +2     
  Lines          6379     6529     +150     
  Branches        584      601      +17     
============================================
+ Hits           5137     5269     +132     
- Misses          854      868      +14     
- Partials        388      392       +4     

Flag	Coverage Δ
integration	80.70% <84.66%> (+0.17%)	⬆️
unit	55.93% <4.00%> (-1.23%)	⬇️

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