* fix(middleware): replace BaseHTTPMiddleware with pure ASGI middleware
BaseHTTPMiddleware wraps streaming responses with receive_or_disconnect
per chunk, blocking the event loop and causing severe throughput
degradation under concurrent streaming load (53% of CPU in profiling).
Converts PrometheusAuthMiddleware to a pure ASGI middleware using the
__call__(scope, receive, send) protocol.
* fix(streaming): remove expensive debug logging and optimize usage stripping
- Remove print_verbose calls that format chunk/response Pydantic objects,
triggering millions of __repr__ calls (8% of CPU in profiling)
- Guard remaining verbose_logger.debug with isEnabledFor(DEBUG) and use
lazy %s formatting instead of f-strings
- Replace usage stripping round-trip (model_dump + delete + reconstruct)
with a _usage_stripped flag, deferring exclusion to serialization time
* fix(proxy): remove per-chunk debug log and use _usage_stripped flag
- Remove verbose_proxy_logger.debug that formatted every streaming chunk
- Honor _usage_stripped flag from streaming handler to exclude usage
during model_dump_json serialization instead of reconstructing objects
* fix(proxy): remove per-chunk debug log in async_data_generator
Remove verbose_proxy_logger.debug that formatted every streaming chunk,
which triggered expensive Pydantic serialization on the hot path.
* fix indentation and add clarifying comment for usage stripping
* fix: guard calculate_total_usage against None usage in chunks
* fix: store chunk copy to preserve usage for calculate_total_usage
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