fix(opencode): strip /v1 from base_url on mid-session /model switch to Anthropic-routed models (#11286)

PR #4918 fixed the double-/v1 bug at fresh agent init by stripping the
trailing /v1 from OpenCode base URLs when api_mode is anthropic_messages
(so the Anthropic SDK's own /v1/messages doesn't land on /v1/v1/messages).
The same logic was missing from the /model mid-session switch path.

Repro: start a session on opencode-go with GLM-5 (or any chat_completions
model), then `/model minimax-m2.7`. switch_model() correctly sets
api_mode=anthropic_messages via opencode_model_api_mode(), but base_url
passes through as https://opencode.ai/zen/go/v1. The Anthropic SDK then
POSTs to https://opencode.ai/zen/go/v1/v1/messages, which returns the
OpenCode website 404 HTML page (title 'Not Found | opencode').

Same bug affects `/model claude-sonnet-4-6` on opencode-zen.

Verified upstream: POST /v1/messages returns clean JSON 401 with x-api-key
auth (route works), while POST /v1/v1/messages returns the exact HTML 404
users reported.

Fix mirrors runtime_provider.resolve_runtime_provider:
- hermes_cli/model_switch.py::switch_model() strips /v1 after the OpenCode
  api_mode override when the resolved mode is anthropic_messages.
- run_agent.py::AIAgent.switch_model() applies the same strip as
  defense-in-depth, so any direct caller can't reintroduce the double-/v1.

Tests: 9 new regression tests in tests/hermes_cli/test_model_switch_opencode_anthropic.py
covering minimax on opencode-go, claude on opencode-zen, chat_completions
(GLM/Kimi/Gemini) keeping /v1 intact, codex_responses (GPT) keeping /v1
intact, trailing-slash handling, and the agent-level defense-in-depth.

hermes_cli/model_switch.py

@@ -727,6 +727,22 @@ def switch_model(
     if not api_mode:
         api_mode = determine_api_mode(target_provider, base_url)
 
+    # OpenCode base URLs end with /v1 for OpenAI-compatible models, but the
+    # Anthropic SDK prepends its own /v1/messages to the base_url.  Strip the
+    # trailing /v1 so the SDK constructs the correct path (e.g.
+    # https://opencode.ai/zen/go/v1/messages instead of .../v1/v1/messages).
+    # Mirrors the same logic in hermes_cli.runtime_provider.resolve_runtime_provider;
+    # without it, /model switches into an anthropic_messages-routed OpenCode
+    # model (e.g. `/model minimax-m2.7` on opencode-go, `/model claude-sonnet-4-6`
+    # on opencode-zen) hit a double /v1 and returned OpenCode's website 404 page.
+    if (
+        api_mode == "anthropic_messages"
+        and target_provider in {"opencode-zen", "opencode-go"}
+        and isinstance(base_url, str)
+        and base_url
+    ):
+        base_url = re.sub(r"/v1/?$", "", base_url)
+
     # --- Get capabilities (legacy) ---
     capabilities = get_model_capabilities(target_provider, new_model)