Add relational query support for MongoDB connector

[codedmart]

Summary

This branch adds initial relational query support to the MongoDB connector, including query planning, pipeline generation, execution, schema exposure, and connector capabilities.

What Changed

• Added relational query execution endpoints to the connector
• Introduced relational pipeline planning/building in mongodb-agent-common
• Added support for:
  • projection
  • filtering
  • sorting
  • pagination
  • joins
  • aggregates
  • window functions
  • unions
  • streaming relational queries
• Exposed relational query capabilities from the connector
• Updated schema generation for relational mode so nested/object-like values are surfaced as string-backed JSON-compatible fields
• Added configuration support and CLI wiring for enabling relational mode
• Added unit and integration test coverage for relational planning/execution paths

Notes

• This is a substantial feature branch that touches connector capabilities, schema generation, execution, configuration, and test infrastructure.
• Relational mode changes how nested MongoDB values are represented in schema/results to better fit SQL-style consumers.
• The implementation also includes optimization work such as predicate pushdown, right-join normalization, and early $match extraction.

Validation

• Added relational pipeline builder tests
• Added relational expression translation tests
• Added relational integration tests for joins, aggregates, windows, and unions