fix(skippy): avoid duplicate lifecycle token handoff

[i386]

Problem

The public #1193 squash (915e6918) introduced a regression where a request can carry pending tokens that were already delivered through the lifecycle commit_generation path. The proposal path then forwards those same tokens a second time. That duplicates the plugin's generated-token prefix and can make a valid proposal session fail closed.

Fix

• Track the generated-token prefix already committed for each request/session.
• Before proposal lookup, commit only an uncommitted suffix.
• Reject rewinds and inconsistent pending-token counts instead of corrupting plugin state.
• Add regression tests covering lifecycle-delivered tokens, missing state, rewinds, and inconsistent suffixes.

This is intentionally based on public main after #1193 and contains no private Mesh architecture or Cacheline code.

Validation

• cargo fmt --all -- --check
• cargo test -p mesh-native-serving-plugin-host --lib (24 passed)
• cargo clippy -p mesh-native-serving-plugin-host --lib --all-targets -- -D warnings

This draft PR only addresses duplicate proposal handoff state. Benchmark integration and session-header wiring are intentionally out of scope.

[coderabbitai]

📝 Walkthrough

Walkthrough

ActivePlugin::propose now validates tracked generation state before committing pending proposal tokens. It rejects invalid counts, missing state, rewound prefixes, and mismatched suffixes. Tests cover these paths and prevent recommitting lifecycle-delivered tokens.

Changes

Proposal generation-state validation

Layer / File(s)	Summary
Generation-state checks and validation tests crates/mesh-native-serving-plugin-host/src/lib.rs	ActivePlugin::propose uses checked token-count subtraction, requires tracked generation state, rejects rewound prefixes, and commits only matching pending suffixes. Tests cover validation failures and no-recommit behavior.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Possibly related PRs

• Mesh-LLM/mesh-llm#1081: Introduces the linear-proposal execution path that ActivePlugin::propose now validates.
• Mesh-LLM/mesh-llm#1149: Introduces the native serving plugin proposal path refined by this change.
• Mesh-LLM/mesh-llm#1180: Modifies proposal handling and token-buffer state management in the same implementation file.

Suggested labels: experimental

Suggested reviewers: michaelneale, ndizazzo

🚥 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.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: preventing duplicate lifecycle token handoff.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch agent/upstream-proposal-state-handoff

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[github-actions]

This pull request is currently a draft. Reviews will not take place until the PR is marked as ready for review.

[coderabbitai]

🧹 Nitpick comments (1)

crates/mesh-native-serving-plugin-host/src/lib.rs (1)

920-1044: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Add coverage for committed counts below prompt counts.

Lines 472-475 reject this input before generation-state lookup. Add a proposal test with committed_token_count < prompt_token_count and assert that the error contains "precedes prompt token count".

Proposed test

+    #[test]
+    fn proposal_rejects_committed_count_before_prompt_count() {
+        let (active, _) = test_support::fake_active_with_events(Duration::ZERO);
+        let error = active
+            .propose(
+                LinearProposalQuery::new(
+                    1,
+                    2,
+                    2,
+                    1,
+                    1,
+                    8,
+                    Instant::now() + Duration::from_millis(100),
+                )
+                .with_pending_token_ids(vec![4].into_boxed_slice()),
+            )
+            .unwrap_err();
+        assert!(error.to_string().contains("precedes prompt token count"));
+    }

🤖 Prompt for 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.

In `@crates/mesh-native-serving-plugin-host/src/lib.rs` around lines 920 - 1044,
Add a test alongside the existing proposal validation tests that initializes a
generation with a prompt token count higher than the query’s
committed_token_count, calls Active::propose, and asserts the returned error
contains "precedes prompt token count". Ensure the case exercises the early
count validation before generation-state lookup.

🤖 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.

Nitpick comments:
In `@crates/mesh-native-serving-plugin-host/src/lib.rs`:
- Around line 920-1044: Add a test alongside the existing proposal validation
tests that initializes a generation with a prompt token count higher than the
query’s committed_token_count, calls Active::propose, and asserts the returned
error contains "precedes prompt token count". Ensure the case exercises the
early count validation before generation-state lookup.

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 35691800-bf97-4497-bf18-f8d273077398

📥 Commits

Reviewing files that changed from the base of the PR and between 915e691 and 9548567.

📒 Files selected for processing (1)

• crates/mesh-native-serving-plugin-host/src/lib.rs