fix(agent): reapply provider headers after model switch (#61099)

[AlexFucuson9]

Summary

switch_model() in agent/agent_runtime_helpers.py rebuilds _client_kwargs from scratch (only api_key + base_url) but does not call _apply_client_headers_for_base_url(). This drops provider-specific HTTP headers like OpenRouter's HTTP-Referer and X-Title, causing subsequent requests to show "Unknown" in OpenRouter dashboard logs.

Fixes #61099

Root Cause

In switch_model() (line ~1903), _client_kwargs is rebuilt:

agent._client_kwargs = {
    "api_key": effective_key,
    "base_url": effective_base,
}

But _apply_client_headers_for_base_url(effective_base) is never called, so headers like:

{"HTTP-Referer": "https://hermes-agent.nousresearch.com", "X-Title": "Hermes Agent"}

are lost. The client is then created without attribution headers.

During initial agent setup (__init__), _apply_client_headers_for_base_url IS called. But after any /model switch, the headers are gone.

Fix

Add agent._apply_client_headers_for_base_url(effective_base) after rebuilding _client_kwargs and before _create_openai_client().

Impact

• Affects all providers that use URL-based attribution (OpenRouter, NVIDIA NIM, Copilot, Kimi)
• After a model switch, all subsequent requests lose provider-specific headers
• OpenRouter dashboard shows "Unknown" instead of "Hermes Agent"

[teknium1]

Salvaged and merged in #61732 (rebase-merge — your commit landed on main with full authorship). Root cause was exactly as you diagnosed: switch_model() was the one client-rebuild site that skipped _apply_client_headers_for_base_url(). We added 3 regression tests on top (OpenRouter attribution, Kimi User-Agent sentinel, stale-header clearing) — 2 of 3 fail on the unpatched code, confirming the bug live. Thanks @AlexFucuson9!