Issue

Addresses Issue #4638

Overview

Added account-level configuration to control how array fields are merged when combining stored requests with incoming requests.

Configuration

New field in account configuration:

account_defaults:
  stored_request: 
    array_merge: "concat"

Modes:

• replace (default): Arrays are replaced following RFC 7386 JSON Merge Patch semantics
• concat: Arrays are concatenated for specific fields (bcat, badv)

Behavior

When array_merge: concat is enabled, array fields are concatenated during stored request merging instead of being replaced. Deduplication is not performed.

This applies to:

• Stored bid request merge
• Default stored request merge

Edge cases preserved:

• Empty arrays in incoming request still clear stored request arrays (RFC 7386 semantics)
• Non-array fields always follow RFC 7386 merge rules
• Single-sided arrays (only in stored or only in incoming) are preserved as-is

Example:

// stored request
{
    "bcat": ["IAB25", "IAB26"],
    "badv": ["competitor.com"],
    "site": {"name": "StoredApp"}
}

// incoming request:
{
    "bcat": ["IAB8"],
    "badv": ["spam.com"],
    "site": {"name": "MyApp"}
}

// result with array_merge: "replace" (default):
{
    "bcat": ["IAB8"],
    "badv": ["spam.com"],
    "site": {"name": "MyApp"}
}

// result with array_merge: "concat":
{ 
    "bcat": ["IAB25", "IAB26", "IAB8"],
    "badv": ["competitor.com", "spam.com"], 
    "site": {"name": "MyApp"}
}