Buildkite MCP Server can now wait for a build to finish

Agents no longer need to hand-roll a polling loop over get_build to find out how a build went. The Buildkite MCP Server now includes wait_for_build, a tool that waits for a build to reach a terminal state and reports the outcome.

Each call waits up to 45 seconds. If the build settles in that window, the tool returns the final build details along with the state it reached: passed, failed, canceled, skipped, not_run, or blocked. If the build is still running, the call comes back with the current state and build_elapsed_seconds, counted from the build's own start time, and the agent calls again to keep waiting. A long wait becomes a handful of cheap calls rather than a tight polling loop, and the tool tells the agent to stop after roughly ten consecutive calls and report the build as still running instead of waiting forever.

This works well ahead of get_build_failure_summary: wait for the build, and if it failed, go straight to the likely cause.

These improvements apply to both the open-source MCP server and the Remote MCP server.