test: fix two flaky tests that fail intermittently in CI

[SmittieC]

Product Description

No change — test-only fixes.

Technical Description

Two tests fail intermittently in CI:

• test_remove_start_end_nodes asserted on pipeline.node_set.all()[0] / [1], but Node has no Meta.ordering, so Postgres returns rows in arbitrary heap order after remove_all_start_end_nodes deletes and rewrites rows. Replaced the positional asserts with a set comparison of flow_ids, which is what the test actually meant to check.
• test_prune_stale_clients slept real wall-clock time (0.4s + 0.1s) against a 0.5s stale timeout, so on a slow runner both clients went stale and got pruned (assert 0 == 1). The sleeps are gone; the test now backdates the first client's timestamp past the timeout, making it deterministic.

Docs and Changelog

• This PR requires docs/changelog update

🤖 Generated with Claude Code

[coderabbitai]

📝 Walkthrough

Walkthrough

Two tests were revised for deterministic behavior. The pipeline test now compares the remaining node flow_id values without relying on query ordering. The Langfuse client pruning test directly backdates one client timestamp instead of waiting with time.sleep, leaving the second client fresh before pruning.

Estimated code review effort: 1 (Trivial) | ~3 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title accurately summarizes the main change: fixing two flaky CI tests.
Description check	✅ Passed	The description matches the template well and includes the required product and technical summaries plus the changelog checkbox.

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