feat(loadtesting): add runId to updateLoadTest for run-level tagging

[sourabhd-cbu]

Index-only. Exposes an optional runId on the updateLoadTest capability so the agent can tag a specific run instead of the test definition.

Why it's needed

The backend (load-testing-backend PR #2845) now accepts runId on updateLoadTest — when set, config.tags is applied to that run's own tags (job.meta.labels) and the test is untouched. But the generic invokeEndpoint strictly rejects any body field not declared in the capability (bind.ts: "unknown arguments are an error"), so without declaring runId here the MCP would throw unknown body: runId before the request reached the backend. This index change is what lets the MCP pass runId through.

Change

• updateLoadTest body gains optional runId (string): "Tag a specific RUN instead of the test definition; only config.tags may accompany it; omit to edit the test."
• Guidance: run-vs-test tag semantics, override-and-inherit (a run inherits the test's tags until set; setting overrides and replaces wholesale — send the union to add).

Follow-up to the merged #400. Backend counterpart: browserstack/load-testing-backend #2845.

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