fix(agent-core-v2): restore turn-start model fallback

[elkaix]

Requirement or Bug

Resolve #351

Bug Reproduction Steps

Bind a session to a model, then delete that model (or its provider) from the config. The next turn fails with an unresolved-model error. Before #323 it switched to a ready model.

Root Cause

#323 (upstream reconcile) removed ensureResolvableModel from AgentProfileService.resolveModelContext, together with the ModelFallbackSwitched event, the model_fallback_triggered telemetry event, and the ranking policy. This PR restores them in the current llm-adapter layout. Fundamental fix.

Code Changes

• llm-adapter/model/default-model-policy.ts: rankDefaultModelCandidates, ported unchanged from the pre-chore: upstream reconcile through reference 99eaa993b #323 kosong/model/defaultModelPolicy.ts.
• profileService.ts: resolveModelContext(turnId) runs ensureResolvableModel at turn bind. If the bound alias does not resolve, it switches to the first ranked candidate that does resolve, then emits ModelFallbackSwitched, a model-fallback WarningIssued, and model_fallback_triggered telemetry. If no candidate resolves, nothing changes.
• llmRequesterService.ts: passes turnId when it builds the per-turn config. This is the only caller that passes it. Compaction and credential lookups stay read-only.
• profileOps.ts and telemetry/events.ts: event and telemetry definitions restored.
• Tests: test/agent/profile/modelFallback.test.ts ports the five pre-chore: upstream reconcile through reference 99eaa993b #323 cases. Three of them fail without the fix. Two test DI setups get IModelService and IProviderService stubs.

Not restored: the global default-model fallback (settleDefaultModel). As the issue comment says, #323 made default_model never be rewritten, so bringing that back is a product decision.

Behavior Changes and Affected Users

Behavior	Before	After	Who relies on the old behavior	Escape hatch
Turn start with a bound model that no longer resolves	Turn fails	Switches to the best-ranked ready model, with a warning and an event	None found. This restores the 2.0-era behavior from #299	Pick the model again with /model
New observable event turn.model_fallback.switched	Not emitted	Emitted on a switch	Not durable, so no wire or state manifest change (regenerated, no drift)	None needed

Affected module: agent-core-v2 profile. Full suites pass: agent-core-v2 6498/6498, agent-gateway 1407/1407.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue (external PRs: issue must have a maintainer's /approve).
• I have added tests that prove my feature works.
• The behavior-change table above is complete, and every removed behavior or flipped default is named in the changeset and either has an escape hatch or was explicitly approved by a maintainer in this PR.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

[pkg-pr-new]

pnpm dlx https://pkg.pr.new/@pymodel/pythinker-code@2c7716d

npx https://pkg.pr.new/@pymodel/pythinker-code@2c7716d

commit: 2c7716d

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📥 Commits

Reviewing files that changed from the base of the PR and between 81f1070 and 2c7716d.

📒 Files selected for processing (9)

• .changeset/restore-turn-model-fallback.md
• packages/agent-core-v2/src/agent/llmRequester/llmRequesterService.ts
• packages/agent-core-v2/src/agent/profile/profileOps.ts
• packages/agent-core-v2/src/agent/profile/profileService.ts
• packages/agent-core-v2/src/app/telemetry/events.ts
• packages/agent-core-v2/src/llm-adapter/model/default-model-policy.ts
• packages/agent-core-v2/test/agent/profile/modelFallback.test.ts
• packages/agent-core-v2/test/session/agentLifecycle/agentLifecycle.test.ts
• packages/agent-core-v2/test/session/subagent/subagentScopeCache.test.ts

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