Revert "Use native JSON bytes for upstream eval traffic"

[xeophon]

Reverts #1798

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Note

Medium Risk
Changes how all upstream eval bodies are encoded and parsed, which can differ from the reverted path on edge cases (non-finite floats, JSON wire shape) even though it restores prior behavior.

Overview
Reverts the native JSON bytes path in EvalClient and restores httpx’s built-in JSON for upstream eval and aux relays.

_request again uses build_request(..., json=body) instead of manually serializing with pydantic_core.to_json, setting content-type, and walking the body to reject non-finite floats. Response handling switches from from_json(resp.content) back to resp.json() in get_response and relay_aux. Unused math and pydantic_core imports are removed.

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Note

Revert native JSON serialization in EvalClient to use httpx defaults

Reverts the EvalClient in eval.py to use httpx's built-in JSON handling instead of pydantic_core. Removes manual pre-validation of non-finite floats and explicit content-type headers, replacing pydantic_core.to_json/from_json calls with json=body and resp.json() throughout. Risk: non-finite float values (e.g. NaN, Infinity) in request bodies are no longer rejected before serialization.

^{Macroscope summarized e0552d4.}

[macroscopeapp]

Approvability

Verdict: Approved

This revert removes a recent JSON serialization optimization and returns to httpx's standard built-in JSON handling. The changes are mechanical and restore previously working behavior.

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