trace: preserve parent context in no-op tracer

[sb123sb123]

Fixes #4027

Cause

The deprecated go.opentelemetry.io/otel/trace.NewNoopTracerProvider only preserved its own nonRecordingSpan wrapper. When a recording parent span was present in the context, it replaced that span with an empty no-op span and dropped the parent trace ID.

Fix

Preserve any existing non-recording span. For a recording parent, create a non-recording span carrying the parent's SpanContext, so no-op instrumentation does not turn a valid trace into a new root.

Tests

• Added TestNoopTracerStartPreservesRecordingParent.
• go test -run '^TestNoopTracerStartPreservesRecordingParent$' -count=1 . (pass)
• go test -count=20 -run '^TestNoopTracerStartPreservesRecordingParent$' . (pass)
• go test ./... from the trace workspace (pass)
• go vet ./... from the trace workspace (pass)
• gofmt -d trace/noop.go trace/noop_test.go and git diff --check (pass)
• /g/OSS-PR-200/work/golangci-lint-6807.exe run --fast-only --timeout 60s --allow-serial-runners from the trace workspace (pass; 0 issues)
• The same regression test fails on the unchanged default branch at 7430d856c5cb42acf07385036965189ae18d70ab.

Limitations

• make precommit could not run because make is unavailable on the Windows host.
• The whole-repository all-module test was attempted, but uncached github.com/go-logr/logr@v1.4.4 could not be downloaded because the host's proxy/GitHub network was intermittent; the complete trace workspace passed.
• go test -race could not run: the Windows host requires CGO for race mode, and its available clang rejects the required -mthreads target option.

No generated files or dependencies were changed.

AI assistance

This PR was prepared with AI assistance by OpenAI Codex under autonomous user direction. The agent performed and reviewed the repository research, implementation, regression test, and verification; the change is intentionally limited to the bug fix, deterministic test, and required changelog entry.

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