Make Git.Files pagination tests reliable under Deno #948

Make the Deno Git-observation pagination test reliable so an unchanged, healthy revision does not intermittently block main and downstream PRs.

The affected case is GO5 — pages are bounded, complete, and about one observation / defaults to 100 records, follows every page once, and honors a larger limit in packages/git/tests/git-observations.test.ts.

Evidence

• Exact unchanged commit: 726275be8ad4ed40ea121aae5d1f527281e61a53.
• Runtime: Deno v2.9.5, GitHub Actions ubuntu-24.04, test-deno (8/15).
• Attempt 1 failed. The test raised:

Error: ENOENT: no such file or directory, readdir '/tmp/xmd-generated-a33iHq/checkout/.git/objects/d7'

• Attempt 2 passed, including this exact GO5 case and the required green check, after rerunning failed jobs with no code change. Main still pointed to the same commit when verified.
• Complete logs from both attempts are preserved in the delivery packet under main-health-726275be-20261018T2116Z/.

The missing directory is inside the generated fixture's Git object database. The cause of its disappearance is not established. The successful rerun demonstrates intermittence; it does not establish a fix.

Additional runtime evidence from PR #952

The same GO5 case failed on the unchanged Slice head
73d3c2aa2b677a580ccd9af7eb9eb218ce2673ab, parent 726275be, in
CI run39217233006 attempt1:

• Node, test-node (5/10):
  ENOENT: no such file or directory, scandir '/tmp/xmd-generated-jztRJY/checkout/.git/objects/de'.
• Bun, test-bun (5/5):
  ENOENT: no such file or directory, scandir '/tmp/xmd-generated-Tcz3en/checkout/.git/objects/ff'.

Both complete first logs were preserved before any rerun under
942/ci-39217233006-attempt1/ in the delivery packet. This occurrence is
not confined to Deno. Slice changes no Git package files; adding its test file
changes the weighted shard assignment, so differing shard names identify the
same existing test. One focused local Deno run of the Git-observations test at
this head passed without a code change; this does not prove cross-runtime
recovery or resolve the cause.

A source trace identifies an unguarded recursive readdir after a directory
inspection in the object-database containment walk. The actor removing the
fanout remains unproven. Any eventual correction must preserve the containment
and read-only guarantees in the acceptance below; no proposed tolerance is
accepted solely on this diagnosis. No repair is included in PR #952.

Acceptance

• Identify why this test can attempt to read a missing fixture object directory and correct the responsible lifetime or filesystem behavior.
• Preserve the 1002-path pagination coverage and the Git read-only guarantees; do not hide the failure by ignoring missing paths or weakening the assertions.
• Add focused evidence that fails for the identified cause and passes with the correction.
• Verify the affected test with deno task test packages/git/tests/git-observations.test.ts and the relevant CI runtime. The shard can be run with deno run --allow-all --frozen scripts/runtime-tests.ts deno 8/15 for the recorded corpus.

Related main-health issue: #947. This flake issue remains open until the cause is fixed and regression evidence is recorded; the green rerun only restores main's health.