test(adapter): add finalize_body helper + 4-corner truth-table tests

[antigenius0910]

Adds a unit test for the reset && !keep_full_text && answer_start > 0 branch at the end of stream_prompt_blocks — the inline if-else that re-prepends the session-reset notice when send-once trimming would otherwise drop it.

Background — this addresses item #3 from my follow-up review on PR openabdev/openab#1115: that branch is the kind of edge case (reset && !streaming && answer_start > 0) that's easy to break silently, and it had no dedicated test.

What changes

• Extracts the 4-line inline branch in stream_prompt_blocks into a pure helper pub fn finalize_body(reset, keep_full_text, answer_start, body) -> String. No behavior change — same truth table, same call site, same conditions.
• Adds 4 tests covering every corner of (reset, keep_full_text, answer_start > 0):
  • reset + send-once + tool ran → notice re-prepended (the case from review item docs(slack): note reply_to bypass + clarify no-native-stream comment #3)
  • reset + send-once + no tools → pass through (notice already in slice via answer_start == 0)
  • reset + keep_full_text → pass through (slice already contains notice)
  • no reset (both flag combos) → pass through (no notice to manage)

Verification

• cargo build passes on top of 78fdee2
• cargo test passes — all 4 new tests green, no existing tests broken

Why a PR-to-PR

Layering this on top of openabdev#1115 rather than opening against main so it ships with the feature it tests. Merge into feat/slack-streaming-toggle and it'll flow into openabdev#1115 automatically. Or pass entirely — the test gap is a nit, not a blocker.