Add shared expression engine tests (field paths, nesting, invalid paths)

[eerxuan]

Summary

Add shared expression engine tests that cover parser/evaluator mechanics common to all operators. These should be tested once with a representative operator, not duplicated per-operator.

Per TEST_COVERAGE.md section 3, these are "Expression Engine Tests (under expressions/)" that test shared behavior.

Tests to Add

1. Field Path Resolution

File: expressions/test_field_path_resolution.py

• Simple: $a on {a: 1}
• Nested object: $a.b on {a: {b: 1}}
• Deep nested: $a.b.c.d on {a: {b: {c: {d: 1}}}}
• Array index: $a.0.b on {a: [{b: 1}, {b: 2}]}
• Composite array traversal: $a.b on {a: [{b: 1}, {b: 2}]}
• Nested arrays: $a.b on {a: [[{b: 1}], [{b: 2}]]}
• Non-existent path: $missing → null
• Non-existent nested: $x.y.0 on {}

2. Expression Nesting

File: expressions/test_expression_nesting.py

• Expression in object: {a: {$abs: -1}}
• Expression in array: [{$abs: -1}]
• Nested expressions: {$add: [{$abs: "$x"}, {$abs: "$y"}]}
• Deep nesting: {$add: [1, {$abs: {$ceil: {$sqrt: "$x"}}}]}
• Complex mixed: {a: [{$add: [1, {$abs: "$x"}]}, {b: {$ceil: "$y"}}]}

3. Invalid Field Paths

File: expressions/test_invalid_field_paths.py

• Empty path: $
• Empty component: $a..b
• Trailing dot: $a.
• Null bytes in path
• $$ prefix errors (undefined variables)
• Dollar prefix in field name: $invalid

Tasks

• Add test_field_path_resolution.py
• Add test_expression_nesting.py
• Add test_invalid_field_paths.py
• Add smoke tests for field path resolution

Test Location

documentdb_tests/compatibility/tests/core/operator/expressions/

Notes

• Use a representative operator (e.g., $add or $project) — these test the expression engine, not operator-specific logic
• Per-operator tests should NOT duplicate these; they should only test operator-specific behavior (type acceptance, $missing handling, argument count, etc.)
• Some per-operator tests currently include invalid field path tests — those can be migrated here over time