fix: SG-42241: Address comment in PR 1200 (#1236)

### Summarize your change.

Address comments on
#1200 (reference)

Small readability cleanups. Drop redundant `int()` around `math.floor`,
rename f to session_file, and replace a for/write loop with writelines.

### Describe the reason for the change.

These are changes that make the code more readable and follow a better
design. No functionality change.

### Describe what you have tested and on which operating system.

Tested on MacOS to see that previews are still working and being
generated.

Signed-off-by: chenl1 <ling.jie.chen@autodesk.com>

Author: chenlj1

src/plugins/rv-packages/session_manager/local_thumbnail_gen.py

@@ -242,7 +242,7 @@ def _pick_frames(self, start_frame: int, end_frame: int) -> list[int]:
         if n_frames < MAX_FILMSTRIP_FRAMES:
             return list(range(start_frame, end_frame + 1))
         incr = (n_frames - 1) / (MAX_FILMSTRIP_FRAMES - 1.0)
-        return [int(math.floor(start_frame + i * incr)) for i in range(MAX_FILMSTRIP_FRAMES)]
+        return [math.floor(start_frame + i * incr) for i in range(MAX_FILMSTRIP_FRAMES)]
 
     def _write_filmstrip_session(
         self, session_path: Path, media_path: str, frames: list[int], width: int, height: int
@@ -257,8 +257,8 @@ def _write_filmstrip_session(
         rhs = " ".join('"defaultLayout"' for _ in frames)
         top_nodes = f'"defaultLayout" {lhs}'
 
-        with open(session_path, "w") as f:
-            f.write(
+        with open(session_path, "w") as session_file:
+            session_file.write(
                 textwrap.dedent(f"""\
                 GTOa (3)
 
@@ -344,9 +344,8 @@ def _write_filmstrip_session(
             """)
             )
 
-            for frame in frames:
-                f.write(
-                    textwrap.dedent(f"""\
+            session_file.writelines(
+                textwrap.dedent(f"""\
                     sourceGroup{frame:06d} : RVSourceGroup (1)
                     {{
                         ui
@@ -381,7 +380,8 @@ def _write_filmstrip_session(
                     }}
 
                 """)
-                )
+                for frame in frames
+            )
 
         return output_width, output_height