Return indexes as list instead of map in API responses

[qodo-code-review]

Description

• The previous implementation returned indexes as a map/dictionary keyed by index name, which was inefficient and required clients to perform lookups
• A code review suggested returning indexes as a list instead, with the index name included as a property on each index object
• This change aligns the API design with REST best practices and simplifies client-side consumption

Deliverables

• Protobuf Update: Change ListIndexesResponse.indexes from map<string, Index> to repeated Index and add name field to Index message
• Service Implementation: Update UserIndexReadsideService to populate the name field and convert indexes to a list-based response
• Test Updates: Modify all test assertions to work with list-based indexes using LINQ methods instead of dictionary methods
• Documentation Fix: Correct query example field name in help documentation

[linear]

DB-1797