test(mongodb): align the in-process _since test with the inclusive filter (#1840)

[angela-helios]

Fixes #1840.

main has been red since #1796: Test Rust fails mongodb_integration_sof_since_filter on every push. That covers #1810, #1822 and #1818, plus PRs branched from them, such as #1839. It is deterministic, not a flake.

#1796 (for #1707) made the in-process runner's _since inclusive, >=, to match the SQL runners and the native Mongo runner ($gte). This MongoDB integration test still assumed the old exclusive filter: it took the cutoff from since-obs-before's own stored timestamp and expected that resource to be excluded. With an inclusive filter, a resource updated exactly at the cutoff is kept, so the assertion fails every time.

Change

The behavior stays as #1796 defined it; only the test changes.

• Exclusion case: the cutoff is now the first write's timestamp plus one millisecond, the precision MongoDB stores. That is the first instant that excludes it, and the write after it, 5 ms later, is still kept.
• New boundary case: a cutoff equal to the first write's own timestamp keeps both resources. This pins the inclusive semantics in the MongoDB suite as well.

The future-cutoff case is unchanged.

Verification

• The MongoDB test binary compiles (cargo test -p helios-persistence --features mongodb --test mongodb_tests --no-run).
• Clippy on helios-persistence, all targets and features, is clean.
• The suite itself needs Docker, which is not available here, so this PR's Test Rust run is the check.