feat(mcp): adopt mcp__server__tool naming convention

Port from anomalyco/opencode#33533. Native MCP tools now register as
mcp__<server>__<tool> (double-underscore delimiter) instead of
mcp_<server>_<tool>, aligning with the convention used by Claude Code,
Codex, and OpenCode.

The double-underscore delimiter disambiguates the server/tool boundary
even when either component contains underscores (the single-underscore
form was ambiguous, which is why is_mcp_tool_parallel_safe already had to
track provenance in a side-map). It also unifies native registration with
the Anthropic-OAuth wire form (_MCP_TOOL_PREFIX = 'mcp__'), so the
single->double promotion that path performed is now a no-op for native
tools while still handling legacy replayed names.

- tools/mcp_tool.py: add MCP_TOOL_NAME_PREFIX + mcp_prefixed_tool_name()
  helper; route _convert_mcp_schema, utility schemas, refresh stale-set,
  and the parallel-safe prefix gate through it
- agent/transports/codex_event_projector.py: mirror convention in the
  deterministic call_id input for MCP server-executed tool calls
- tests: update produced-name assertions to the new convention

tests/test_toolsets.py

@@ -133,9 +133,9 @@ class TestValidateToolset:
     def test_mcp_alias_uses_live_registry(self, monkeypatch):
         reg = ToolRegistry()
         reg.register(
-            name="mcp_dynserver_ping",
+            name="mcp__dynserver__ping",
             toolset="mcp-dynserver",
-            schema=_make_schema("mcp_dynserver_ping", "Ping"),
+            schema=_make_schema("mcp__dynserver__ping", "Ping"),
             handler=_dummy_handler,
         )
         reg.register_toolset_alias("dynserver", "mcp-dynserver")
@@ -144,7 +144,7 @@ class TestValidateToolset:
 
         assert validate_toolset("dynserver") is True
         assert validate_toolset("mcp-dynserver") is True
-        assert "mcp_dynserver_ping" in resolve_toolset("dynserver")
+        assert "mcp__dynserver__ping" in resolve_toolset("dynserver")
 
 
 class TestGetToolsetInfo: