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.

run_agent.py

@@ -1674,12 +1674,26 @@ class AIAgent:
         turn-scoped).
         """
         import logging
+        import re as _re
         from hermes_cli.providers import determine_api_mode
 
         # ── Determine api_mode if not provided ──
         if not api_mode:
             api_mode = determine_api_mode(new_provider, base_url)
 
+        # Defense-in-depth: ensure OpenCode base_url doesn't carry a trailing
+        # /v1 into the anthropic_messages client, which would cause the SDK to
+        # hit /v1/v1/messages.  `model_switch.switch_model()` already strips
+        # this, but we guard here so any direct callers (future code paths,
+        # tests) can't reintroduce the double-/v1 404 bug.
+        if (
+            api_mode == "anthropic_messages"
+            and new_provider in ("opencode-zen", "opencode-go")
+            and isinstance(base_url, str)
+            and base_url
+        ):
+            base_url = _re.sub(r"/v1/?$", "", base_url)
+
         old_model = self.model
         old_provider = self.provider