feat: reporting-api group_by description

[mkleina]

Description

Document the new group_by query parameter for the Proxy Reporting API GET /v1/usage endpoint. When set to subaccount, the response includes usage broken down by sub-account with an additional username field in each record.

Changes

• feature: Add group_by query parameter definition to the OpenAPI spec
• feature: Add username field to the UsageRecord schema (present only when group_by=subaccount)
• feature: Add named response examples ("Default usage" and "Grouped by sub-account") for clearer API documentation

Related changes

• Backend implementation: massivebackend handler at lambdas/proxy/reporting/internal/api/handler.go (already merged)

How Has This Been Tested?

Ran mintlify dev locally and verified:

• The group_by parameter renders correctly as a string field with description
• The response examples panel shows both "Default usage" (without username) and "Grouped by sub-account" (with username) as selectable options
• No OpenAPI validation errors related to the reporting spec