refactor(gmi): move User-Agent to profile.default_headers

The previous revision of this PR added six GMI-specific branches
(`elif base_url_host_matches(..., 'api.gmi-serving.com')`) across
run_agent.py and agent/auxiliary_client.py, plus a _HERMES_UA_HEADERS
constant in auxiliary_client.py.

ProviderProfile already has a `default_headers: dict[str, str]` field
commented as 'Client-level quirks (set once at client construction)'.
Other plugins (ai-gateway, kimi-coding) already use it. Two of the four
auxiliary_client sites we previously patched already had a generic
`else: profile.default_headers` fallback that picked it up (so did
both run_agent sites).

This revision:

* Sets `default_headers={'User-Agent': 'HermesAgent/<ver>'}` on the
  GMI profile in plugins/model-providers/gmi/__init__.py.
* Reverts all six GMI-specific branches in run_agent.py and
  auxiliary_client.py.
* Adds the generic profile-fallback `else` block to the two
  auxiliary_client sites (`_to_async_client`, `resolve_provider_client`)
  that didn't have it yet. This benefits every provider whose profile
  declares default_headers, not just GMI — e.g. Vercel AI Gateway's
  HTTP-Referer/X-Title now flow through the async client path too.
* Replaces the GMI-specific URL-branch tests with a profile-level
  assertion and keeps the run_agent integration test (with
  `provider='gmi'` so the fallback picks up the profile).

Net diff vs main: +82/-0 across 5 files, touching only the GMI plugin,
two generic fallback blocks in auxiliary_client.py, AUTHOR_MAP, and
tests. No core files change.

Based on #20907 by @isaachuangGMICLOUD.

plugins/model-providers/gmi/__init__.py

@@ -1,5 +1,6 @@
 """GMI Cloud provider profile."""
 
+from hermes_cli import __version__ as _HERMES_VERSION
 from providers import register_provider
 from providers.base import ProviderProfile
 
@@ -12,6 +13,10 @@ gmi = ProviderProfile(
     env_vars=("GMI_API_KEY", "GMI_BASE_URL"),
     base_url="https://api.gmi-serving.com/v1",
     auth_type="api_key",
+    # Attribution so GMI can identify traffic from Hermes Agent.
+    # The generic profile.default_headers fallback in run_agent.py and
+    # agent/auxiliary_client.py picks this up at client construction time.
+    default_headers={"User-Agent": f"HermesAgent/{_HERMES_VERSION}"},
     default_aux_model="google/gemini-3.1-flash-lite-preview",
     fallback_models=(
         "zai-org/GLM-5.1-FP8",