Mock provider request matching breaks when new keys are added to input toArray()

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Update Summary: [One-line status update for stakeholders]
Short Description: Mock provider request matching breaks when new keys are added to input toArray()
Check-in Date: MM/DD/YYYY
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Problem/Motivation

Needed for #3546954: Normalize file api's to allow providers to use them.

The EchoProvider's getMatchingRequest() method in ai_test compares the current request against stored YAML fixtures using strict JSON equality (Json::encode($request['request']) === Json::encode($array), line 406 of EchoProvider.php). When a new key with a default value is added to an input type's toArray() (e.g. remote_files added to ChatInput), all existing YAML test fixtures break because they do not contain the new key - even though the default value is functionally irrelevant.

This means every time a new property with a default value is added to any input type, all existing test YAML files for that operation type must be manually updated. This is fragile and creates unnecessary churn.

Proposed resolution

• Change the comparison in EchoProvider::getMatchingRequest() so that it automatically fills in default values for keys that are present in the current input's toArray() but missing from the stored YAML request, before comparing.
• This way existing YAML fixtures continue to work even after new keys with default values are added to input types.
• One approach: iterate the keys of $array (the current input) and for any key missing in $request['request'], add the default value from the input before comparison. Alternatively, only compare the keys that exist in the YAML fixture (subset matching) rather than requiring exact equality.

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