fix(server): restore worktree paths on git without worktree list -z

[jaikhuranna]

Closes #12761

What Changed

git worktree list --porcelain -z fails on git < 2.36 (-z was added in 2.36), and the failure was swallowed by allowNonZeroExit: true — so on older git, every branch silently lost its worktree path in listRefs. This PR retries with the line-delimited porcelain when -z is rejected and parses that form.

• parseWorktreeBranchPaths now takes the field separator (\0 for -z, \n for plain porcelain) — the record structure is identical.
• The newline test is now capability-aware: on git ≥ 2.36 it asserts the newline worktree path round-trips; on older git it asserts the ref surfaces with worktreePath: null (the line-delimited form cannot represent newline paths — the truncated path is filtered by the existing stat check).
• A new mocked-spawner test covers the fallback deterministically on any git version: -z rejected with 129, plain porcelain succeeds, worktree path resolves.
• The coalescing test now expects 2 worktree scans per snapshot on old git (rejected -z + fallback) instead of 1.

Why

The suite failed on machines with git 2.34 (e.g. Ubuntu 22.04): preserves newline characters in worktree paths when listing refs asserted a worktree path the driver could never produce there. Root cause is the silent -z failure above — the fallback restores worktree paths for all normal cases on older git, and the tests now document the newline limitation honestly instead of assuming modern git.

Stacked on #11729 (this branch is based on it; the diff will shrink to only these changes once that merges).

UI Changes

N/A — no UI changes.

Checklist

• This PR is small and focused
• I explained what changed and why
• N/A — no UI changes
• N/A — no animation/interaction changes

Verification: full src/vcs suite 95/95 pass on git 2.34.1 (previously 93/94); tsc --noEmit clean for the server package.

Done with GLM (fireworks-ai/glm-flash-latest) via the OpenCode harness in T3 Code.