fix: use StdioServerParameters to bypass Agno executable whitelist

Agno's MCPTools has an undocumented executable whitelist that blocks
gitea-mcp (Go binary). Switch to server_params=StdioServerParameters()
which bypasses this restriction. Also fixes:

- Use tools.session.call_tool() for standalone invocation (MCPTools
  doesn't expose call_tool() directly)
- Use close() instead of disconnect() for cleanup
- Resolve gitea-mcp path via ~/go/bin fallback when not on PATH
- Stub mcp.client.stdio in test conftest

Smoke-tested end-to-end against real Gitea: connect, list_issues,
create issue, close issue, create_gitea_issue_via_mcp — all pass.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/conftest.py

@@ -20,6 +20,8 @@ except ImportError:
 for _mod in [
     "airllm",
     "mcp",
+    "mcp.client",
+    "mcp.client.stdio",
     "mcp.registry",
     "telegram",
     "telegram.ext",