fix(libsy): allow message_hash_fallback with classify_trigger = user_turn

[deepujain]

What

Relaxes the classifier config validation so message_hash_fallback is accepted with classify_trigger = "user_turn" in addition to "new_session". Only "every_request" is still rejected: it retains no target, so a fallback identity has nothing to key on. Docs updated in docs/reference/toml_schema.md, docs/routing_algorithms/llm_classifier_routing.md, and crates/switchyard-server/README.md. Adds a regression test covering the accepted and rejected combinations.

Why

Fixes the runtime half of #495. With user_turn and no session id, every turn is classified, so user_turn behaves like every_request for callers that send no session id. message_hash_fallback was meant to fix exactly that by keying the retained target on a hash of the first user message, but config validation rejected the combination. This also revives the runtime change from #519, which was closed for inactivity with only the docs update outstanding; the requested docs are included here.

Notes for reviewers

Start at affinity_router in crates/libsy/src/algorithms/llm_class.rs: the runtime already supports the combination, so this change only lifts the spurious validation rejection in the three validate sites. New test: message_hash_fallback_accepts_retaining_triggers. Local: cargo fmt clean, clippy -p switchyard-libsy clean, cargo test -p switchyard-libsy 314 passed.

Summary by CodeRabbit

• New Features

  • message_hash_fallback now supports classification on both new sessions and user turns.
  • Routing retains targets using the first user message hash when no session ID is available.
  • The setting remains disabled by default.

• Bug Fixes

  • Configuration validation consistently accepts the supported triggers and rejects unsupported request-level triggering.

• Documentation

  • Updated configuration references, routing guidance, and server documentation to reflect the expanded trigger support.

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📥 Commits

Reviewing files that changed from the base of the PR and between 06e88c0 and bb9cf35.

📒 Files selected for processing (4)

• crates/libsy/src/algorithms/llm_class.rs
• crates/switchyard-server/README.md
• docs/reference/toml_schema.md
• docs/routing_algorithms/llm_classifier_routing.md

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Walkthrough

The change expands message_hash_fallback support to NewSession and UserTurn. Validation and route construction still reject EveryRequest. Tests and documentation reflect the updated rule.

Changes

Message Hash Fallback

Layer / File(s)	Summary
Classifier validation and route construction crates/libsy/src/algorithms/llm_class.rs	Task, custom, and shared classifier paths accept NewSession and UserTurn. Tests verify both accepted triggers and continued rejection of EveryRequest.
Trigger requirement documentation crates/switchyard-server/README.md, docs/reference/toml_schema.md, docs/routing_algorithms/llm_classifier_routing.md	Documentation now states that message_hash_fallback supports new_session and user_turn.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~15 minutes

Merge Risk: ⚪ Minimal · up to bb9cf

The classifier now supports message-hash fallback for user turns while continuing to reject every-request classification; validation, routing, tests, and documentation are aligned, with no actionable merge-blocking risk identified.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: allowing message_hash_fallback with classify_trigger = user_turn.
Docstring Coverage	✅ Passed	Docstring coverage is 83.33% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 6 functions across 1 files. (3 skipped: 3 u…
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.

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_{A rabbit checks the trigger gate
New sessions hop beside user turns
Every request stays outside
Hashes guide the route with care
Green tests nibble at the rule}

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

[afourniernv]

Looks good. This carries forward the runtime change from #519. Ayush had already reviewed that implementation and only asked for the routing guide and server README updates: #519 (review). Both are included here.

@ryan-lempka tagging you since this sits on the classify_trigger/AffinityRouter path you added. Holding the merge until the v0.3 freeze is lifted.

[deepujain]

Thanks for the review. Noted that the merge is on hold until the v0.3 freeze lifts.

[afourniernv]

Thanks for your contribution @deepujain !