test(beego): add middleware unit tests

[prathamesh-sonpatki]

Summary

• Rebases Beego instrumentation onto current main
• Adds 13 unit tests covering the Middleware() function

Tests cover

• Span is created for each request
• Span name format: METHOD /path
• HTTP attributes: http.request.method, url.path, server.address
• Response status code attribute (http.response.status_code)
• 5xx responses mark span as Error; 4xx and 2xx do not
• SpanKind is Server
• Incoming W3C traceparent header creates a child span with matching trace ID
• Requests with no incoming context produce a root span
• Downstream next handler receives the request context containing the active span

Test plan

• go test ./instrumentation/beego/... — all 13 tests pass
• golangci-lint run ./instrumentation/beego/... — clean

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