Response Agent Fix

[jdchawla29]

Makes request for response agent without the trace-id header as it is not an agent step, instead uses instrumentation to emit a custom span.
Fixes HUD-546

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Note

Instrument and header handling updates

• Instruments ResponseAgent.determine_response with @instrument(category="agent", name="response_agent", internal_type="user-message") to emit a custom span
• Sends chat completion with extra_headers={"Trace-Id": ""} so this request does not carry a trace id
• Updates HTTP instrumentation in hud/eval/instrument.py to add trace headers only if not already present (case-insensitive) for both httpx and aiohttp
• Extends hud.telemetry.instrument to accept internal_type and include it in emitted spans

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