Implement Deletes for FlatPostgresCollection

[suddendust]

PR: Implement Delete Operations for Flat PostgreSQL Collections

Methods Implemented

• delete(Key key) - Delete single document by primary key
• delete(Set keys) - Bulk delete by multiple keys
• delete(Filter filter) - Delete using legacy Filter (delegates to new API)
• delete(query.Filter filter) - Delete using new query Filter API
• deleteAll() - Delete all documents from collection

Why Deprecate delete(Filter filter)?

The legacy Filter class uses hardcoded JSONB document access patterns that don't work with flat collections where fields are direct PostgreSQL columns. The parsing logic assumes nested document structure.

Why New delete(query.Filter filter)?

The new query.Filter wraps FilterTypeExpression which:

• Works with the v1 PostgresQueryParser that supports pluggable field transformers
• Uses FlatPostgresFieldTransformer to correctly resolve field names to PostgreSQL columns
• Properly handles nested JSONB paths via JsonIdentifierExpression
• Maintains consistency with other query operations that already use this API

[codecov]

Codecov Report

❌ Patch coverage is 84.53039% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.65%. Comparing base (6a2b05d) to head (ca5fd33).

Files with missing lines	Patch %	Lines
...nsformer/LegacyFilterToQueryFilterTransformer.java	85.00%	9 Missing and 9 partials ⚠️
...documentstore/postgres/FlatPostgresCollection.java	83.33%	8 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #270      +/-   ##
============================================
+ Coverage     80.52%   80.65%   +0.13%     
  Complexity     1396     1396              
============================================
  Files           239      240       +1     
  Lines          6695     6871     +176     
  Branches        615      638      +23     
============================================
+ Hits           5391     5542     +151     
- Misses          900      914      +14     
- Partials        404      415      +11     

Flag	Coverage Δ
integration	80.65% <84.53%> (+0.13%)	⬆️
unit	53.26% <0.55%> (-1.41%)	⬇️

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