COBS-1350: Add Insights traces query support

[srajputdo]

Summary

Adds a godo client for the public Insights traces query endpoints, matching the OpenAPI spec merged in digitalocean/openapi#1256.

• InsightsService.SearchTraces(ctx, region, *TracesSearchRequest) → POST /v2/insights/query/{region}/traces/search
• InsightsService.GetTrace(ctx, region, traceID, *TraceGetOptions) → GET /v2/insights/query/{region}/traces/{trace_id} with optional from / to lookup window
• Request/response types mirroring the spec (camelCase tags): TracesSearchRequest, TracesTimeRange / TracesTimeBound, TracesFilterExpression, TracesFilterField, TracesFilterValue, TracesOrderBy, TracesPaginationRequest, TracesSearchResponse, TraceSummary, TracesPaginationResponse, TraceGetOptions, Trace, TraceSpan
• Type-discriminated filter grammar (condition / and / or / not / text_search) with operators eq / neq / gt / gte / lt / lte / in / exists
• TracesFilterValue encodes the typed-wrapper value form ({"type":"string","value":...}) and accepts the bare scalar/array form on decode
• InsightsTraces* constants for filter node types, operators, scopes, sort directions, value types, and trace status
• SearchTraces + GetTrace checks added to examples/insights/smoke/main.go

Plain-text error bodies from the traces service (400, 404 trace not found, 502) surface as ErrorResponse.Message via the existing CheckResponse fallback; covered by tests.

Testing

Rebased on main after #1156 (spans) merged; both clients coexist.

Unit / CI-equivalent — gofmt -s clean, go vet ./... clean, go test -race ./... passes on Go 1.23.12, 1.24.9 (CI matrix) and 1.27. New tests: TestInsights_SearchTraces, TestInsights_SearchTracesMinimalRequest, TestTracesFilterValueMarshal, TestTracesFilterValueUnmarshal, TestInsights_SearchTracesEscapesRegion, TestInsights_SearchTracesError, TestInsights_GetTrace, TestInsights_GetTraceNilOptions, TestInsights_GetTraceEscapesPath, TestInsights_GetTraceNotFound.

Live — examples/insights/smoke passes the traces checks on staging (s2r1) and prod (nyc2). A broader live matrix driven through this client against both environments: staging 59 passed / 2 skipped (no multi-span or second-region data), prod 61 passed, covering:

Area	Cases
SearchTraces request	relative, offset (now-168h), absolute RFC3339, absolute epoch-seconds time ranges; minimal body; summary field decoding
Pagination	limit 1 (hasMore=true), 0 (server default), 1000
orderBy	started asc/desc and durationNs desc verified sorted; serviceName, traceId, errorCount, spanCount, status, aliases; multiple clauses; omitted direction
Filters	eq, neq, in (string_array), exists, gt/gte/lt/lte, bool value, empty type → condition, resource/attributes/span.attributes scopes, text_search, nested and/or/not, no-match → empty data
GetTrace	nil options; span count / error count match the search summary; spans ordered by start; parent links (prod: 14 spans, 1 root, 13 linked); relative, RFC3339 and epoch-ms windows
Errors surfaced as *ErrorResponse	400 (plain text): from > to, range > 168h, limit 1001 / -1, missing timeRange, bad orderBy field / direction, not with 2 children, in with scalar; 404 (plain text): unknown, uppercase, out-of-window and wrong-region trace IDs; 404 / 401 (gateway JSON): unknown region, bad token

Note: the Insights query routes do not currently return the standard ratelimit-* headers (also true for logs search), so Response.Rate is empty for these calls. Tracking separately; no client change needed.

Jira: https://do-internal.atlassian.net/browse/COBS-1350