Adds guardrail.litellm_content_filter.pre_mcp_call.blocks: against the real
Datadog MCP server, a content_filter guardrail configured mode=pre_mcp_call
blocks a banned keyword in an MCP tool call's arguments with HTTP 400 attributed
to the pre_mcp_call hook, and lets a clean argument reach the upstream server.
The guardrail attaches with default_on because per-key/request guardrail
selection is dropped from the synthetic MCP request the hook sees; the banned
keyword is unique per run so default_on only intercepts this test's own call.
mode must be pre_mcp_call - a pre_call config silently no-ops on tools/call
because the event type is rewritten for call_mcp_tool.
Drives the tool directly via /mcp-rest/tools/call for a deterministic check of
the same pre_mcp_call enforcement the OpenAI-SDK chat path hits when a model
invokes an MCP tool.
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