perf: avoid small own clones and intermediate Vec allocations

- Derive Copy on 6 small enums (MessageRole, TaskState, StorageKind, EmbeddingBackend, PdfIngestMode, KnowledgeEntityType)
- Change create_ingestion_payload files param from Vec<FileInfo> to &[FileInfo]
- Remove 5 intermediate Vec allocations (4 embedding serialization + 1 format_history) using write! loop
- Remove 7 unnecessary .clone() calls exposed by Copy derive

common/src/utils/config.rs

@@ -3,7 +3,7 @@ use serde::Deserialize;
 use std::env;
 
 /// Selects the embedding backend for vector generation.
-#[derive(Clone, Deserialize, Debug, Default, PartialEq)]
+#[derive(Clone, Copy, Deserialize, Debug, Default, PartialEq)]
 #[serde(rename_all = "lowercase")]
 pub enum EmbeddingBackend {
     /// Use OpenAI-compatible API for embeddings.
@@ -15,7 +15,7 @@ pub enum EmbeddingBackend {
     Hashed,
 }
 
-#[derive(Clone, Deserialize, Debug, PartialEq)]
+#[derive(Clone, Copy, Deserialize, Debug, PartialEq)]
 #[serde(rename_all = "lowercase")]
 pub enum StorageKind {
     Local,
@@ -33,7 +33,7 @@ fn default_s3_region() -> String {
 }
 
 /// Selects the strategy used for PDF ingestion.
-#[derive(Clone, Deserialize, Debug)]
+#[derive(Clone, Copy, Deserialize, Debug)]
 #[serde(rename_all = "kebab-case")]
 pub enum PdfIngestMode {
     /// Only rely on classic text extraction (no LLM fallbacks).