Merge-only bulk upsert of Complete/total session quotas.

Semantics:

• Merge only: omitted services are unchanged (not a replace-all)
• Batch size: session_quotas must contain 1–500 rows
• Clear: set both full_sessions_limit and total_sessions_limit to null
• When numbers: both integers ≥ 0, and full_sessions_limit ≤ total_sessions_limit
• Unknown service names return 400 and apply nothing
• Duplicate (service_name, environment) returns 400
• Mixed environments in one request are allowed
• Non-enterprise accounts return 403

Auth: requires throttling:write (operator keys). Reader/v1 keys receive 403.

Example:
PUT /throttling/session-quotas with { "session_quotas": [{ "service_name": "checkout-api", "environment": "production", "full_sessions_limit": 4, "total_sessions_limit": 10 }] }