fix: handle absolute paths correctly with tempFileDir upload option (#14436)

### What?

This PR fixes `tempFileDir` configuration not respecting absolute paths
when `useTempFiles` is enabled for uploads.

### Why?

1. **Absolute paths ignored**: Setting `tempFileDir: '/tmp'` would still
create temp files under the working directory (e.g.,
`/Users/project/tmp/...`) instead of `/tmp/...`

2. **Path duplication**: The default `tempFileDir` could result in
duplicated working directory segments in the path

### How?
Uses `path.resolve()` to handle both cases:
- Absolute paths (e.g., `/tmp`) are returned as-is
- Relative paths (e.g., `tmp`) are resolved against `process.cwd()`

Fixes #12910

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Co-authored-by: Jessica Chowdhury <jessica@trbl.design>