fix(plugins): preserve virtual server context for tool calls

[Tethys0]

Summary

Fixes #6233.

Tool plugin GlobalContext.server_id was not reliable for virtual-server calls. The normal invocation fallback discarded the explicit virtual-server ID, while the existing-context path could overwrite it with the tool's gateway ID. That made PluginCondition.server_ids and custom server-scoped plugin logic silently miss every call routed through a virtual server.

This patch:

• preserves the explicit virtual-server server_id through both tool-context construction paths;
• keeps an existing middleware context ID when no virtual-server scope is present;
• uses the tool gateway ID only as the fallback when no virtual-server ID exists.

Testing

• uv run --frozen pytest -q tests/unit/mcpgateway/services/test_tool_service_tenant_id.py tests/unit/mcpgateway/services/test_tool_service_coverage.py - 383 passed, 1 skipped (intentional pre-existing coverage marker)
• uv run --frozen ruff check mcpgateway/services/tool_service.py tests/unit/mcpgateway/services/test_tool_service_tenant_id.py tests/unit/mcpgateway/services/test_tool_service_coverage.py - passed
• uv run --frozen ruff format --check mcpgateway/services/tool_service.py tests/unit/mcpgateway/services/test_tool_service_tenant_id.py tests/unit/mcpgateway/services/test_tool_service_coverage.py - passed
• git diff --check - passed

The full make test target was also started locally but did not complete within the available run window; no full-suite result is claimed here.