fix: a pull request snapshot without a file count shows none

[Tryanks]

Summary

A Bitbucket or Azure DevOps summary has no changed-file count, so the snapshot carried changed_files: 0 and the PR page's Files tab read "Files 0" until a files read answered. The count is now optional end to end, as #690 made additions and deletions.

 crates/core/src/pull_request.rs     PullRequestSnapshot.changed_files: u64 -> Option<u64>, #[serde(default)]
 crates/protocol/src/lib.rs          wire note above PROTOCOL_VERSION (no bump)
 crates/services/src/github/…        Some(changedFiles)          (unchanged value)
 crates/services/src/forgejo/reads   Some(changed_files)         (unchanged value)
 crates/services/src/gitlab/reads    Some(changes_count)         (unchanged value)
 crates/services/src/bitbucket/reads 0 -> None
 crates/services/src/azure/reads     0 -> None
 crates/ui/src/pull_requests/detail  files_label(): files read's count, else the snapshot's, else plain "Files"

Readers of the snapshot's count: only the Files tab label (detail.rs). The row detail in pull_requests/mod.rs shows the line stat, not the file count; the watch fingerprint (pull_request_watch.rs) does not include it; nothing sorts by it. Once the files read answers, the tab shows the read's PullRequestFiles.changed_files, which the label already preferred; no new state.

Evidence

• Before: Bitbucket/Azure snapshot changed_files: 0 → tab "Files 0" (reported in feat: Bitbucket Cloud pull requests #696 and feat: Azure DevOps pull requests #702).
  After: PR page on an Azure DevOps PR read anonymously (public PowerShell/PSGallery-Cache-Repo Frosted Instrument: full visual redesign on macOS vibrancy #53, linked by discovery from a checkout on its branch; fresh TCODE_DATA_DIR, throwaway HOME, HOSTNAME="Demo Mac", English, light). The tab reads "Files":

  

Tests:

• tcode-core pull_request::tests::a_kept_snapshot_reads_its_counts (was a_kept_snapshot_reads_its_line_counts, extended): a literal snapshot kept before this change, with changed_files: 2, still reads Some(2); one without the field reads None. Before this change the type was u64, so a missing field could not read as absent at all; with Option<u64> serde already reads a missing field as None, so the serde(default) matches feat: GitLab merge requests as pull requests #690's form rather than being load-bearing.
• tcode-ui pull_requests::detail::tests::a_snapshot_without_a_file_count_labels_the_files_tab_without_one: a linked snapshot without a count labels the tab "Files"; once the files read answers (150), it reads "Files 150". gpui's test context cannot read rendered text, so the label moved from a closure in switch into PullRequestView::files_label, which switch renders.

Checks (macOS):

• cargo fmt --all --check: clean
• cargo clippy --workspace --all-targets --locked -- -D warnings: Finished, no warnings
• cargo nextest run --workspace --locked: 1156 tests run: 1156 passed, 13 skipped
• cargo machete: no unused dependencies

Windows/Linux, mobile and Web: CI. Phone and dark theme not attached (desktop light only for this fix).

Merge Danger

Door: two-way
Blast Radius: small
The wire's changed_files may now be null. Client and host build from one tree between releases (Principle 9). A snapshot kept before this change still loads.

Closes #698