fix(libsy): keep tier prompts and handoff notes on a same-format hop

[sabhatinas]

Tier system prompts and handoff notes never reached the model on a same-format hop.

Why. A codec stores the verbatim inbound body at decode, the algorithm then mutates the canonical IR only, and at encode the codec replays that stored body whenever one exists for the target format — returning a snapshot taken before the addition, without ever reading the IR. All three buffered codecs do this, so Anthropic→Anthropic and Responses→Responses dropped it too. Cross-format usually misses the lookup, but not always: preservation carries bodies embedded by earlier hops, so a multi-hop request can hit the same short-circuit.

Fix. Drop the preserved bodies once something has been added, which sends the codec down its normal encode path. Two call sites and one helper. The whole map is cleared, not just the inbound entry — the addition is missing from every stored body equally.

Verified against the ticket's reproduction with a stub upstream: <NO SYSTEM MESSAGE> on main, the configured prompt on this branch. No new tests — the existing fixtures now seed a preserved body, so the tests already covering each mutation assert what happens to it, and fail on unpatched code.

Fixes SWITCH-1224

[coderabbitai]

Walkthrough

The change adds request fingerprint seals to preserved LLM requests. Decoders seal normalized requests, and exact replay now requires a current seal. Regression tests cover mutations from prompts, notes, and router-added instructions.

Changes

Preserved Request Freshness

Layer / File(s)	Summary
Request fingerprint contract crates/protocol/src/llm.rs	PreservationMetadata stores an optional request seal. LlmRequest seals request state and detects changes to the serialized request shape.
Translation replay freshness crates/switchyard-translation/src/codecs/*/buffered.rs, crates/switchyard-translation/src/util.rs, crates/switchyard-translation/tests/extension_points.rs	Request decoders seal normalized requests. exact_preserved_request rejects stale requests before replay.
Replay invalidation coverage crates/libsy/src/algorithms/util/prompts.rs, crates/switchyard-translation/tests/request_translation.rs	Tests cover configured prompts, conversation notes, same-format mutations, and router-added instructions across wire formats.

Estimated code review effort: 4 (Complex) | ~45 minutes

Poem

I’m a rabbit with a seal in my hat,
Fresh requests stay exactly at that.
If notes hop in line,
Replay says, “Not mine!”
New instructions march out—just like that.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
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.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main fix: preserving tier prompts and handoff notes during same-format request hops.

---

_{Comment @coderabbitai help to get the list of available commands.}

[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@crates/protocol/src/llm.rs`:
- Around line 351-354: Correct the freshness documentation around
Self::seal_preservation to match the fingerprint behavior: remove “rewriting the
model” from the changes that invalidate preservation, or explicitly state that
model changes are excluded.

🪄 Autofix

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: 5f270b69-4ebc-47df-82cc-2a1a7afb2932

📥 Commits

Reviewing files that changed from the base of the PR and between 759658e and a845afc.

📒 Files selected for processing (8)

• crates/libsy/src/algorithms/util/prompts.rs
• crates/protocol/src/llm.rs
• crates/switchyard-translation/src/codecs/anthropic/buffered.rs
• crates/switchyard-translation/src/codecs/openai_chat/buffered.rs
• crates/switchyard-translation/src/codecs/responses/buffered.rs
• crates/switchyard-translation/src/util.rs
• crates/switchyard-translation/tests/extension_points.rs
• crates/switchyard-translation/tests/request_translation.rs