postprocess: switch from gemini-3-pro-preview to gemini-3-flash-preview for the default (#1535)

`flash` is cheaper and faster and seems to still be sufficiently
capable.

~~I did have to adjust the prompt slightly to explicitly say to include
the surrounding Rust Markdown code block. This isn't actually used right
now, but it is in #1534, so it seems worth preserving.~~

Edit: The prompt is kept the same now.

Author: kkysen

c2rust-postprocess/postprocess/__init__.py

@@ -52,7 +52,7 @@ def build_arg_parser() -> argparse.ArgumentParser:
         "--model-id",
         type=str,
         required=False,
-        default="gemini-3-pro-preview",
+        default="gemini-3-flash-preview",
         help="ID of the LLM model to use (default: gemini-3-pro-preview)",
     )
 

c2rust-postprocess/tests/llm-cache/CommentTransfer/partition/bf27c23abbb2f5c2d7dba1891b748252983959f5dc232f35146250b6d2d1fb29/metadata.toml

@@ -1,8 +1,7 @@
 transform = "CommentTransfer"
 identifier = "partition"
-model = "gemini-3-pro-preview"
-response = """```rust
-/// Lomuto Partition Scheme:
+model = "gemini-3-flash-preview"
+response = """/// Lomuto Partition Scheme:
 /// Partitions the array so that elements < pivot are on the left, 
 /// and elements >= pivot are on the right.
 #[no_mangle]
@@ -32,8 +31,7 @@ pub unsafe extern "C" fn partition(
         arr.offset(high as isize) as *mut ::core::ffi::c_int,
     );
     return i + 1 as ::core::ffi::c_int;
-}
-```"""
+}"""
 
 [[messages]]
 role = "user"

c2rust-postprocess/tests/llm-cache/CommentTransfer/partition/bf27c23abbb2f5c2d7dba1891b748252983959f5dc232f35146250b6d2d1fb29/response.txt

@@ -1,4 +1,3 @@
-```rust
 /// Lomuto Partition Scheme:
 /// Partitions the array so that elements < pivot are on the left, 
 /// and elements >= pivot are on the right.
@@ -29,5 +28,4 @@ pub unsafe extern "C" fn partition(
         arr.offset(high as isize) as *mut ::core::ffi::c_int,
     );
     return i + 1 as ::core::ffi::c_int;
-}
-```
+}

c2rust-postprocess/tests/llm-cache/CommentTransfer/quickSort/d124abd1fe6752700e122e17288aac1b78606f3ffefcb6dc2c59e103933d1411/metadata.toml

@@ -1,6 +1,6 @@
 transform = "CommentTransfer"
 identifier = "quickSort"
-model = "gemini-3-pro-preview"
+model = "gemini-3-flash-preview"
 response = """```rust
 #[no_mangle]
 pub unsafe extern "C" fn quickSort(