fix(session): anchor message windows on renderable tail rows

[ai-ag2026]

Summary

• Anchors paginated /api/session message windows on the newest renderable non-tool row when the raw tail only contains hidden tool result rows.
• Adds regression coverage for tail windows, msg_before, and all-tool fallback behavior.

Why

Tool result rows are not rendered as standalone transcript rows. A long session whose newest raw rows are only tool outputs can otherwise open to a non-empty API message array but a blank visible transcript. The backend should choose a tail window that contains the newest visible transcript anchor while preserving raw message offsets for paging.

Test Plan

• python3 -m py_compile api/routes.py tests/test_session_message_window_renderable_tail.py
• python3 -m pytest tests/test_session_message_window_renderable_tail.py -q

Result: 4 passed.

[nesquena-hermes]

Holding briefly — anchors message windows on renderable tail rows. +112 LOC in api/routes.py. The fix concept is right (compression boundary should be the anchor, not the visible tail) and pairs conceptually with #2915 (already shipped in v0.51.137 / Release DI).

What needs checking:

1. Does this conflict with fix: marker-based compression anchor calculation #2915's marker-based anchor logic that just shipped? Both touch the same conceptual surface from different ends.
2. The _renderable_tail_anchor function — does it agree with the existing compression_anchor_visible_idx semantics or does it redefine them?
3. ai-ag2026's flood-pattern means treating each PR individually with care — confirm this isn't bundling unrelated changes.

Will review more carefully on the next sweep. @ai-ag2026, thanks.

[nesquena-hermes]

Merged in Release DL / v0.51.140 (stage-batch22 — 5-PR hold-bucket reassessment with PRs #2899 #2902 #2964 #2970 #2986).

Thanks @ai-ag2026! 🚢