fix: retry downloads on transient errors instead of breaking (#1398)

## Motivation

`download_file_with_retry()` has a `break` in the generic exception
handler that exits the retry loop after the first transient failure.
This means network timeouts, connection resets, and server errors all
cause an immediate download failure — the two remaining retry attempts
never run.

## Changes

**download_utils.py**: Replaced `break` with logging and exponential
backoff in the generic exception handler, matching the existing
rate-limit handler behavior.

Before:
```python
except Exception as e:
    on_connection_lost()
    if attempt == n_attempts - 1:
        raise e
    break  # exits loop immediately
```

After:
```python
except Exception as e:
    on_connection_lost()
    if attempt == n_attempts - 1:
        raise e
    logger.error(f"Download error on attempt {attempt + 1}/{n_attempts} ...")
    logger.error(traceback.format_exc())
    await asyncio.sleep(2.0**attempt)
```

## Why It Works

The `break` statement was bypassing the retry mechanism entirely.
Replacing it with the same log-and-backoff pattern used by the
`HuggingFaceRateLimitError` handler means all 3 attempts are actually
used before giving up. The exponential backoff (1s, 2s) gives transient
issues time to resolve between attempts.

## Test Plan

### Manual Testing
- Downloads that hit transient network errors now retry instead of
failing immediately

### Automated Testing
- `uv run basedpyright` — 0 errors
- `uv run ruff check` — passes
- `uv run pytest src/exo/download/tests/ -v` — 11 tests pass

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: rltakashige <rl.takashige@gmail.com>

src/exo/download/download_utils.py

@@ -378,10 +378,14 @@ async def download_file_with_retry(
             logger.error(traceback.format_exc())
             await asyncio.sleep(2.0**attempt)
         except Exception as e:
-            on_connection_lost()
             if attempt == n_attempts - 1:
+                on_connection_lost()
                 raise e
-            break
+            logger.error(
+                f"Download error on attempt {attempt + 1}/{n_attempts} for {model_id=} {revision=} {path=} {target_dir=}"
+            )
+            logger.error(traceback.format_exc())
+            await asyncio.sleep(2.0**attempt)
     raise Exception(
         f"Failed to download file {model_id=} {revision=} {path=} {target_dir=}"
     )