fix(RecordExpander): replace deepcopy with shared reference to prevent O(N²) memory amplification

[devin-ai-integration]

Summary

Replaces copy.deepcopy(parent_record) with a direct shared reference in RecordExpander when remain_original_record=True.

Problem: When expanding records from a nested array (e.g., Stripe invoice events with many line items), the previous implementation deep-copied the entire parent record for every expanded child. For a parent with N children, this created N full copies of the parent (which itself contains all N children), resulting in O(N²) memory usage.

This appears to cause sync stalling for high-volume connectors like source-stripe, where invoice events can contain hundreds of line items. With 10 concurrent workers, the memory amplification is further compounded.

Fix: Since original_record is only read from in downstream transformations and then removed (via RemoveFields), deep-copying is unnecessary. All expanded siblings now share the same parent reference, reducing memory from O(N²) to O(N).

Affected code: airbyte_cdk/sources/declarative/expanders/record_expander.py — both _apply_parent_context (dict items) and the inline non-dict branch.

Declarative-First Evaluation

This fix modifies the CDK's RecordExpander component itself. No declarative YAML alternative exists — the RecordExpander is the declarative component, and the performance issue is in its Python implementation.

Test Coverage

• test_record_expander_shares_parent_reference_for_dict_items — verifies all expanded dict records share the same parent reference (identity check with is)
• test_record_expander_shares_parent_reference_for_non_dict_items — same for non-dict items
• test_record_expander_large_expansion_memory_efficient — simulates a Stripe-like scenario with 500 line items and verifies shared references

All 41 tests in test_dpath_extractor.py pass, including 17 pre-existing RecordExpander tests.

Review & Testing Checklist for Human

• Verify that no downstream consumer of original_record mutates it in-place before it is removed. The shared reference is safe as long as original_record is treated as read-only.
• Confirm source-stripe invoice_line_items and subscription_items incremental streams work correctly after this change (they read from original_record then remove it).

Notes

Related to https://github.com/airbytehq/oncall/issues/12136 (Stripe sync stalling after April 15).

The RecordExpander component was introduced in v7.17.0 via #859. The remain_original_record feature was first used by source-stripe v6.0.0 (airbytehq/airbyte#76095) for invoice_line_items and subscription_items incremental streams.

Link to Devin session: https://app.devin.ai/sessions/3e3a626cafe64a60a2ee62fa119d7ce5

[devin-ai-integration]

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[github-actions]

PyTest Results (Fast)

4 043 tests  +3   4 032 ✅ +3   7m 51s ⏱️ +6s
    1 suites ±0      11 💤 ±0 
    1 files   ±0       0 ❌ ±0 

Results for commit 1735375. ± Comparison against base commit 886fcf8.

[github-actions]

PyTest Results (Full)

4 046 tests  +3   4 034 ✅ +3   10m 52s ⏱️ -31s
    1 suites ±0      12 💤 ±0 
    1 files   ±0       0 ❌ ±0 

Results for commit 1735375. ± Comparison against base commit 886fcf8.