Add integration tests for new MCP efficiency tool step attribute

[nikita-phantom-ops]

Adding tests to address the new MCP efficiency_tool functionality with the step attribute added.
Tests include:

• TestMCPEfficiencyStepConsistency: verifies costs and efficiency ratios match within tolerance for stepped and non-stepped results
• TestMCPEfficiencyStepDoesNotOOM: running a real-world test case that we've encountered when a 7d window aggregation results in a OOM
• TestMCPEfficiencyInvalidStep: validates that incorrect values for step are rejected

This PR is a follow up of opencost/opencost#3869

[Copilot]

Pull request overview

This PR adds a new Go integration-test file, test/integration/mcp/efficiency_step_test.go, covering the newly added step attribute on the MCP get_efficiency tool (follow-up to opencost/opencost#3869). It introduces a helper to call the tool over MCP/JSON-RPC and three test suites: step/no-step consistency, large-window OOM resilience, and rejection of invalid step values. It reuses the shared helpers (initializeMCPSession, getMCPURL, MCPResponse, abs) already present in helpers.go.

Changes:

• Adds MCPEfficiencyRequest/MCPEfficiencyData types and callMCPEfficiencyTool to invoke the get_efficiency MCP tool.
• Adds TestMCPEfficiencyStepConsistency, TestMCPEfficiencyStepDoesNotOOM, and TestMCPEfficiencyInvalidStep, plus a compareEfficiencyResults helper with cost/efficiency tolerance checks.
• Note: the new file is not registered in test/integration/mcp/test.bats, so the CI harness (bats -r test/integration) will not execute these tests.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.