fix(serving): bound validation error logging

[ananthsub]

Summary

• replace full-body pretty printing on shared 422 handling with a 4 KiB escaped prefix and exact original byte size
• bound validation error summaries to 20 entries and omit Pydantic's potentially large or secret-bearing input values
• preserve FastAPI's standard 422 status, body, and headers byte-for-byte
• keep logging failure from masking the client response

Correctness and reliability

The request has already crossed FastAPI's validation boundary; this change only alters diagnostics. Empty, JSON, non-JSON, control-character, unavailable-body, and multi-megabyte inputs are covered. Logged prefixes are escaped so request-controlled newlines and nulls cannot inject raw control characters into logs.

Validation

• uv run --extra dev pytest tests/unit_tests/test_server_utils.py -q — 33 passed
• scoped pre-commit — passed

Focused benchmark

Median handler formatting time and traced peak allocation, comparing the previous two full json.dumps(..., indent=4) calls with the bounded logger:

• 1 KiB: 0.039 ms / 0.004 MiB baseline; 0.044 ms / 0.007 MiB candidate
• 2 MiB: 6.822 ms / 6.501 MiB baseline; 0.057 ms / 0.017 MiB candidate
• 8 MiB: 30.243 ms / 26.001 MiB baseline; 0.071 ms / 0.016 MiB candidate

The tiny-body path is effectively unchanged; large-body CPU and allocation no longer scale with body size.

Closes #3007