webdav: support filesystem-backed dead properties

[izouxv]

Introduce FileSystemProps so a FileSystem can provide dead
property holders directly without forcing prop handling through
OpenFile. The lookup now prefers filesystem-level props, falls
back to file-level DeadPropsHolder when the filesystem does not
provide one, and reuses the same helper path for PROPFIND,
PROPPATCH, and property copying during COPY/MOVE.

The tests cover the intended behavior in two layers: prop
handling exercises add/get-all/delete operations against
filesystem-provided props while confirming file-backed dead
props stay hidden and unmodified, and copy behavior verifies
both filesystem-backed property copying and fallback to
file-backed dead properties when no filesystem props are
available.

Fixes golang/go#80232.

[gopherbot]

This PR (HEAD: e32e04b) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/net/+/796460.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
• You need a Gmail or other Google account to log in to Gerrit.
• To change your code in response to feedback:
  • Push a new commit to the branch used by your GitHub PR.
  • A new "patch set" will then appear in Gerrit.
  • Respond to each comment by marking as Done in Gerrit if implemented as suggested. You can alternatively write a reply.
  • Critical: you must click the blue Reply button near the top to publish your Gerrit responses.
  • Multiple commits in the PR will be squashed by GerritBot.
• The title and description of the GitHub PR are used to construct the final commit message.
  • Edit these as needed via the GitHub web interface (not via Gerrit or git).
  • You should word wrap the PR description at ~76 characters unless you need longer lines (e.g., for tables or URLs).
• See the Sending a change via GitHub and Reviews sections of the Contribution Guide as well as the FAQ for details.

[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/796460.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Gopher Robot:

Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
A maintainer will review your change and provide feedback. See
https://go.dev/doc/contribute#review for more info and tips to get your
patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

During May-July and Nov-Jan the Go project is in a code freeze, during which
little code gets reviewed or merged. If a reviewer responds with a comment like
R=go1.11 or adds a tag like "wait-release", it means that this CL will be
reviewed as part of the next development cycle. See https://go.dev/s/release
for more details.

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/796460.
After addressing review feedback, remember to publish your drafts!

[izouxv]

fix: golang/go#80232

[gopherbot]

Message from Dmitri Shuralyov:

Patch Set 1:

(3 comments)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/796460.
After addressing review feedback, remember to publish your drafts!