Local filesystem writes can retain stale tail data when overwriting shorter objects

Problem

The Local FS storage backend opens existing objects with os.O_RDWR|os.O_CREATE but without os.O_TRUNC:

handle, err := os.OpenFile(o.path, os.O_RDWR|os.O_CREATE, 0o644)

fsObject.Put writes from offset zero, so overwriting an existing object with shorter content leaves the old tail in the file. The same behavior affects the uncompressed StoreFile path that uses getHandle(false).

Example:

existing object: abcdefgh
new object:      xy
read result:     xycdefgh

This makes the Local FS backend inconsistent with object-storage backends, where a put replaces the complete object. It can corrupt local snapshots, cache files, or metadata when an object is rewritten with a smaller payload.

Proposed fix

Use truncation for complete-object writes, either by adding os.O_TRUNC to the write handle or by explicitly truncating and resetting the offset before copying:

handle, err := os.OpenFile(o.path, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0o644)
if err != nil {
    return nil, err
}

The truncating mode should only be used for complete replacement writes. Read, range-read, append, and multipart/random-offset paths should retain their existing semantics.

Tests

Please add two separate regression tests:

1. Put path:

   • Write a longer payload to an object.
   • Overwrite it with a shorter payload using Put.
   • Verify Size equals the new payload length.
   • Verify reading returns exactly the new payload, with no stale suffix.

2. Direct, uncompressed StoreFile path:

   • Store a longer source file into an object.
   • Overwrite it with a shorter source file using uncompressed StoreFile.
   • Verify Size equals the new source file length.
   • Verify reading returns exactly the new source content, with no stale suffix.

The compressed StoreFile path is a separate implementation and should not be conflated with this regression.

Relevant code:

• packages/shared/pkg/storage/storage_fs.go
• fsObject.Put
• fsObject.StoreFile
• fsObject.getHandle

Read packages/shared/pkg/storage/storage_fs.go, focusing on fsObject.Put, fsObject.StoreFile, and fsObject.getHandle, then run the storage package tests. Add separate regression coverage for shorter overwrites through Put and uncompressed StoreFile; done means Size and reads contain exactly the replacement payload, while compressed StoreFile and non-replacement paths retain their existing behavior.