Allow GCSToSambaOperator to write to the share root

[MohammadHijjawi97]

GCSToSambaOperator rejects every object copied to the root of the SMB share. The destination containment check added in #67857 compares the resolved path with destination_path + os.sep, so:

• destination_path="/" builds the prefix "//", and "/dir/file.txt" does not start with it;
• destination_path="" or "." normalises to ".", and the resolved relative path ("dir/file.txt") has no "./" prefix.

In both cases the task fails with ValueError: Resolved destination path ... is outside the configured destination_path, although the file would land inside the share. This has affected the operator since samba provider 4.12.6.

This PR handles the share root the same way GCSToSFTPOperator._resolve_destination_path and the Amazon validate_destination_path helper already do:

• for a relative root, it rejects only paths that climb out with .. or that are absolute;
• for any other base, it strips a trailing separator before building the prefix.

Object names containing .. are still refused.

Tests: added parametrized cases for /, "" and . as the destination, plus traversal cases from a relative share root. pytest providers/samba/tests/unit/samba/transfers/test_gcs_to_samba.py: 32 passed; the 3 share-root cases fail without the change. ruff format / ruff check are clean.

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[yuseok89]

Nice catch. The share-root handling looks good to me.

[MohammadHijjawi97]

The failing Non-DB tests: providers job is not related to this change. Every failure and error in it comes from one SQLAlchemy error, first raised while collecting providers/google/tests/unit/google/cloud/hooks/test_stackdriver.py:

When initializing mapper Mapper[AssetWatcherModel(asset_watcher)], expression 'Trigger' failed to locate a name ('Trigger')

The samba hook, hive and common.compat failures are the follow-on "One or more mappers failed to initialize" error in the same pytest session (a samba change pulls in the google tests, so they share it). None of the failures are in test_gcs_to_samba.py.

This is the collection issue fixed on main by #74066, which was merged on Oct 2, after this run. I can rebase onto main so the job runs with that fix.

[MohammadHijjawi97]

Rebased onto main to pick up #74066; no change to the PR's own code.