Preserve filesystem prefixes embedded in paths

[abdulm5]

Fixes #854

Summary

Preserves dbfs: and file: text when it appears inside DBFS, Volume,
or local path names while continuing to normalize leading filesystem
scheme prefixes.

Why

Path wrappers currently use global string replacement to remove filesystem
prefixes. This also removes matching text from legitimate path components,
making paths such as /dbfs: unreachable.

When recursively listing /, a returned /dbfs: directory is normalized
back to / and queued again, causing an infinite traversal loop. Restricting
normalization to the beginning of the path preserves legitimate names and
prevents this aliasing.

What changed

Interface changes

None.

Behavioral changes

• Embedded and trailing dbfs: and file: text is preserved.
• Leading dbfs: and file: scheme prefixes retain their existing behavior.
• Recursive listings no longer revisit the parent when a child is named
  dbfs: or file:.

Internal changes

• Added a shared prefix-only path normalizer.
• Updated DBFS and Volume path construction.
• Made local file: handling prefix-only while preserving Windows file URI
  and UNC behavior.
• Added regression coverage for exact API paths and recursive traversal.

How is this tested?

• tests/test_dbfs_mixins.py: 30 passed.
• Ruff lint: passed.
• Ruff formatting check: passed.
• git diff --check: passed.

[github-actions]

If integration tests don't run automatically, an authorized user can run them manually by following the instructions below:

Trigger:
go/deco-tests-run/sdk-py

Inputs:

• PR number: 1498
• Commit SHA: bdf48436bd599008b557d670e8863a65c4c02968

Checks will be approved automatically on success.