fix(grpc): classify adaptive admission rejections accurately

[DavidBellamy]

Problem

The gRPC router maps every status other than 400, 404, 408/504, and 5xx to internal_error. Enforced adaptive admission returns HTTP 429 when a model fleet is saturated, so ordinary capacity rejections inflate smg_router_request_errors_total{error_type="internal_error"} and look like gateway failures.

Expected behavior

• Classify 429 as a bounded admission rejection, not an internal failure.
• Preserve the HTTP 429 response and Retry-After behavior.
• Keep actual 5xx and unexpected statuses distinguishable.
• Add focused status-mapping tests.

Acceptance criteria

• 429 maps to a stable non-internal metric label.
• Existing 400, 404, timeout, and backend mappings remain unchanged.
• A regression test covers 429 and representative existing statuses.
• No routing or admission decision behavior changes.