fix(legacy-meter): guard MCP alias metering off in tests

Alias tests monkeypatch _get/_post but _meter_legacy_mcp uses _client()
directly, so every pytest run recorded real usage and reset zero-window
retirement streaks. STATEHUB_MCP_LEGACY_METER=off skips metering; set in
tests/conftest.py. Verified meter counts flat across the alias suite.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

mcp_server/server.py

@@ -226,6 +226,8 @@ def _meter_legacy_mcp(*, tool: str | None = None, resource: str | None = None) -
     (`mcp:<tool_name>` / resource URI). Failures never surface to the caller —
     the meter gates retirement, it must not break the aliased call itself.
     """
+    if os.environ.get("STATEHUB_MCP_LEGACY_METER", "on") == "off":
+        return
     if tool:
         key = f"mcp:{tool}"
         replacement = _LEGACY_MCP_TOOL_REPLACEMENTS[tool]