feat: add OpenTelemetry trace spans to all handlers and pipeline functions

Add #[tracing::instrument(skip_all)] to 44 functions:
- 19 API handlers in mod.rs
- 3 chat handlers, 6 DAST handlers, 7 graph handlers
- 2 pipeline orchestrator functions
- 7 scanner functions (sbom, semgrep, gitleaks, cve, lint, patterns)

This generates trace spans for SigNoz visibility into request
latency, scan pipeline stages, and error tracking.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

compliance-agent/src/api/handlers/chat.rs

@@ -17,6 +17,7 @@ use super::ApiResponse;
 type AgentExt = Extension<Arc<ComplianceAgent>>;
 
 /// POST /api/v1/chat/:repo_id — Send a chat message with RAG context
+#[tracing::instrument(skip_all)]
 pub async fn chat(
     Extension(agent): AgentExt,
     Path(repo_id): Path<String>,
@@ -126,6 +127,7 @@ pub async fn chat(
 }
 
 /// POST /api/v1/chat/:repo_id/build-embeddings — Trigger embedding build
+#[tracing::instrument(skip_all)]
 pub async fn build_embeddings(
     Extension(agent): AgentExt,
     Path(repo_id): Path<String>,
@@ -226,6 +228,7 @@ pub async fn build_embeddings(
 }
 
 /// GET /api/v1/chat/:repo_id/status — Get latest embedding build status
+#[tracing::instrument(skip_all)]
 pub async fn embedding_status(
     Extension(agent): AgentExt,
     Path(repo_id): Path<String>,