fix(webapp): fetch run-scoped trace subtrees for large traces (#4024)

Fixed trace rendering for child and nested runs in large traces.
Dashboard and trace API responses now load the requested run's trace
subtree instead of depending on the run span appearing in the initial
trace slice.

docs/management/runs/retrieve-trace.mdx

@@ -2,3 +2,7 @@
 title: "Retrieve run trace"
 openapi: "v3-openapi GET /api/v1/runs/{runId}/trace"
 ---
+
+Returns the OpenTelemetry trace subtree for the run you request. The response `trace.rootSpan` is that run's span — not necessarily the trace-wide root — with its descendant spans nested under `children`.
+
+For a child or nested run inside a large trace, this endpoint scopes the tree to that run so you still get a useful subtree even when the full trace has more spans than the platform can return in one response.

docs/v3-openapi.yaml

@@ -448,7 +448,7 @@ paths:
     get:
       operationId: get_run_trace_v1
       summary: Retrieve run trace
-      description: Returns the full OTel trace tree for a run, including all spans and their children.
+      description: Returns the OTel trace subtree for the requested run — the run's span as `rootSpan`, its ancestor chain, and its descendant spans. For child or nested runs in a large trace, this is scoped to that run rather than the trace-wide root.
       responses:
         "200":
           description: Successful request
@@ -464,7 +464,9 @@ paths:
                         type: string
                         description: The OTel trace ID.
                       rootSpan:
-                        $ref: "#/components/schemas/SpanDetailedSummary"
+                        allOf:
+                          - $ref: "#/components/schemas/SpanDetailedSummary"
+                        description: The requested run's span, with nested descendant spans as `children`. Not necessarily the trace-wide root span.
         "401":
           description: Unauthorized request
           content: