list_blobs (and other XML pagers) deserialize each page body twice

Copilot raised this issue when I was prompting it about the `list_blobs` perf test. ## Summary The generated paging callback for XML list operations deserializes the full response body once just to read the continuation marker, then the body is deserialized a **second** time when the caller iterates items or calls `into_model()`. For large pages this doubles XML parsing work and is measurable in perf testing. ## Where it happens In the generated `list_blobs` callback (`sdk/storage/azure_storage_blob/src/generated/clients/blob_container_client.rs`), the body is parsed to `ListBlobsResponse` only to extract `next_marker`, then the **raw bytes** are stored back into the response: ```rust let (status, headers, body) = rsp.deconstruct(); let res: ListBlobsResponse = xml::from_xml(&body)?; // (1) full parse, only next_marker used let rsp = RawResponse::from_bytes(status, headers, body).into(); // raw bytes re-wrapped Ok(match res.next_marker { Some(next_marker) if !next_marker.is_empty() => PagerResult::More { response: rsp, continuation: PagerContinuation::Token(next_marker), }, _ => PagerResult::Done { response: rsp }, }) ``` Then, when the consumer pulls items, the `Pager`/`ItemIterator` parses the same bytes again via the blanket `Page` impl for `Response<P, F>` in `azure_core`'s `pager.rs`: ```rust async fn into_items(self) -> crate::Result<Self::IntoIter> { let page: P = self.into_model()?; // (2) second full parse of the same body page.into_items().await } ``` The same applies to explicit page iteration, where the caller calls `into_model()` themselves: ```rust let mut pager = client.list_blobs(None)?.into_pages(); while let Some(page) = pager.try_next().await? { let page = page.into_model()?; // (2) second full parse // ... } ``` So the body is parsed at (1) and again at (2) — twice per page. ## Impact - ~2x XML deserialization cost per page for list operations. - Most visible with large page sizes / many items (e.g., `list_blobs`, `list_containers`, `list_blobs_hierarchical`, and likely other XML pagers). - Affects both item iteration and `into_pages()` + `into_model()`. ## Possible fixes 1. **Extract only the marker without a full deserialization** in the callback — e.g., parse just the `NextMarker` element, or read it from a header where available, instead of `xml::from_xml::<ListBlobsResponse>(&body)`. 2. **Carry the already-deserialized model forward** so `into_model()` is a no-op — i.e., have the pager keep the typed `ListBlobsResponse` from step (1) rather than re-wrapping raw bytes, avoiding the second parse. Either approach removes the redundant parse. Option 2 generalizes across all generated XML pagers if the emitter can stash the parsed model instead of raw bytes.