enhance: Return trace ID in REST responses (#49968)

issue: #49967

## What was changed

- Add `X-Milvus-Trace-Id` to REST responses for `/v1/vector/...` and
`/v2/vectordb/...` routes.
- Create a REST entry trace span before access logging so response
headers and REST logs use the same request trace ID.
- Keep the v2 wrapper from overwriting the entry trace ID when one
already exists.
- Add timeout and early-return coverage for the trace ID header.

## Verification

- `gofumpt -l internal/distributed/proxy/httpserver/handler_v2.go
internal/distributed/proxy/httpserver/traceid_test.go
internal/distributed/proxy/httpserver/utils.go
internal/distributed/proxy/httpserver/constant.go
internal/distributed/proxy/httpserver/timeout_middleware.go
internal/distributed/proxy/service.go`
- `git diff --check`
- `zsh -c 'source scripts/setenv.sh && go test -v -count=1 -tags
dynamic,test -gcflags="all=-N -l" -ldflags="-r
${MILVUS_WORK_DIR}/cmake_build/lib -r
${MILVUS_WORK_DIR}/internal/core/output/lib"
./internal/distributed/proxy/httpserver -run
"TestHTTPReturnWritesTraceIDHeader|TestHTTPReturnSkipsTraceIDHeaderWhenMissing|TestTraceIDHandlerFuncCreatesRequestTraceID|TestTraceIDHandlerFuncCoversV1AndV2RestRoutes|TestTraceIDHandlerFuncCreatesTraceIDWhenUnsampled|TestTraceIDHandlerFuncReturnsUniqueTraceID|TestTraceIDHandlerFuncCoversEarlyAbort|TestWrapperPostBadJSONReturnsTraceIDHeader|TestWrapperPostKeepsRequestTraceID|TestTimeoutMiddlewareReturnsTraceIDHeader"
-timeout 300s'`
- Manual curl verification against local standalone: REST response
header and REST access log trace IDs matched; 10 repeated search
requests returned 10 valid and unique 32-hex trace IDs.

## Note

Full `go test ./internal/distributed/proxy/httpserver` currently hits an
existing unrelated `TestSearchV2/search#09` assertion mismatch: expected
`Mismatch type uint8`, actual contains `Mismatch type []uint8 ...`.

Signed-off-by: sijie-ni-0214 <sijie.ni@zilliz.com>