Add verbose syntax tests for $lookup

[danielfrankcom]

This change adds testing for the verbose (let and pipeline) form of $lookup.

It focuses on how let variables are resolved across the sub-pipeline, including how each supported stage recognizes them, how they scope across nested $lookup stages, and how they remain accessible across multi-stage sub-pipelines.

The new organization structure replaces the earlier test_lookup_with_expr.py file.

This is related to #673 and the comments there. Changes based on @PatersonProjects' original PR.

[documentdb-triage-tool]

🤖 Auto-triaged by documentdb-triage-tool.

Applied: compatibility test, enhancement
Project fields suggested: Component test-coverage · Priority P2 · Effort L · Status Needs Review
Confidence: 0.88 (mixed)

Reasoning

component from path globs (test-coverage); effort from diff stats (1590+309 LOC, 6 files); LLM: Adds new test coverage for the verbose $lookup syntax (let/pipeline), expanding compatibility test suite without touching production code.

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