WIP: Add OpContext trace span helpers

[jamesgao-jpg]

Summary

• add milvus::OpContext::trace_span as the trace parent carrier for downstream search paths
• add milvus::OpContext::GetTraceSpan() accessors for trace span reads
• add shared trace helper APIs in common/Tracer.h for Cardinal and Knowhere
• remove the separate common/Trace.h wrapper; callers should include common/Tracer.h directly
• rename the scoped span-slot restoration helper to NestedSpanGuard
• remove thin wrapper helpers SetTemporaryOpContextTraceSpan and StartScopedSpan; callers now construct NestedSpanGuard and ScopedSpan directly
• keep temporary span replacement scoped so the previous span is restored when the guard is destroyed

Dependency context

Earlier branch commit 6476f5068efdea819403dff9cb42f8f9f37f502d was published as milvus-common/1.0.0-6476f50@milvus/dev#96b754f8757ce3c59471b6d8dcbfaba4.

Branch commit 242dedaaa2af86f1c070872cc75cc62e58e1882f was tagged as v1.0.0-242deda, but it is superseded by current branch commit 45bff01293e050be7af2c9c1e5b70c49da8940ef, which moves the helper APIs into common/Tracer.h and removes common/Trace.h. A new tag/package is needed before downstream conan pins can move.

Downstream PRs consuming this helper package:

• Cardinal: https://github.com/zilliztech/cardinal/pull/865
• Knowhere: Use OpContext trace span for Cardinal tracing knowhere#1684
• Milvus: Seed search OpContext trace span milvus-io/milvus#50606

Verification

• git diff --check
• rg -n "common/Trace\\.h|Trace.h" across milvus-common, Knowhere, and Cardinal: no matches
• make test was attempted locally but did not run because conan is not available on PATH in this local environment.