refactor(session-state): key continuations per provider to survive provider switches

Before, every provider stored its opaque continuation id under the
single outbound.db key `sdk_session_id`. Flipping a session's
agent_provider (e.g. Codex → Claude) meant the new provider read the
old provider's id at wake, handed it to its own SDK, and got a
"No conversation found" error that cost the user one sacrificed
message before the stale-session recovery path cleared the id.

This reshapes session_state so continuations are keyed
`continuation:<provider>` instead. Consequences:

- Per-provider continuations coexist. Flipping Claude → Codex → Claude
  resumes the Claude thread exactly where it left off, with the
  intervening Codex thread also still on file.
- No provider ever reads another provider's id. Switching costs no
  sacrificed message and emits no transient error.
- Legacy installs are migrated forward on first startup:
  migrateLegacyContinuation() adopts any pre-existing `sdk_session_id`
  row into the current provider's slot (best guess — it was whichever
  provider ran last), then deletes the legacy row unconditionally so
  it can't poison a future provider's read.

runPollLoop now takes providerName alongside the provider instance,
and threads it through processQuery to setContinuation on init.

Tests: 9 new tests covering set/get isolation across providers,
clear-specificity, legacy-adoption, legacy-always-deleted,
prefer-existing-slot-over-legacy, and idempotency of a second
migration call.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

container/agent-runner/src/db/session-state.test.ts

@@ -0,0 +1,100 @@
+import { beforeEach, describe, expect, test } from 'bun:test';
+
+import { getOutboundDb, initTestSessionDb } from './connection.js';
+import {
+  clearContinuation,
+  getContinuation,
+  migrateLegacyContinuation,
+  setContinuation,
+} from './session-state.js';
+
+beforeEach(() => {
+  initTestSessionDb();
+});
+
+function seedLegacy(value: string): void {
+  getOutboundDb()
+    .prepare('INSERT INTO session_state (key, value, updated_at) VALUES (?, ?, ?)')
+    .run('sdk_session_id', value, new Date().toISOString());
+}
+
+describe('session-state — per-provider continuations', () => {
+  test('set/get round-trip, case-insensitive provider key', () => {
+    setContinuation('claude', 'claude-conv-1');
+    expect(getContinuation('claude')).toBe('claude-conv-1');
+    expect(getContinuation('Claude')).toBe('claude-conv-1');
+    expect(getContinuation('CLAUDE')).toBe('claude-conv-1');
+  });
+
+  test('providers are isolated — switching reads the right slot', () => {
+    setContinuation('claude', 'claude-conv-1');
+    setContinuation('codex', 'codex-thread-xyz');
+
+    expect(getContinuation('claude')).toBe('claude-conv-1');
+    expect(getContinuation('codex')).toBe('codex-thread-xyz');
+  });
+
+  test('clearContinuation only affects the specified provider', () => {
+    setContinuation('claude', 'keep-me');
+    setContinuation('codex', 'drop-me');
+
+    clearContinuation('codex');
+
+    expect(getContinuation('claude')).toBe('keep-me');
+    expect(getContinuation('codex')).toBeUndefined();
+  });
+
+  test('unknown provider returns undefined', () => {
+    expect(getContinuation('never-used')).toBeUndefined();
+  });
+});
+
+describe('session-state — legacy migration', () => {
+  test('adopts legacy value into current provider when current is empty', () => {
+    seedLegacy('old-session-id');
+
+    const adopted = migrateLegacyContinuation('claude');
+
+    expect(adopted).toBe('old-session-id');
+    expect(getContinuation('claude')).toBe('old-session-id');
+  });
+
+  test('always deletes legacy row regardless of migration outcome', () => {
+    seedLegacy('old-session-id');
+    setContinuation('claude', 'existing');
+
+    migrateLegacyContinuation('claude');
+
+    // After migration the legacy key must be gone, whether or not it was adopted.
+    // A subsequent migration for a different provider must not see it.
+    const resultAfterSecondCall = migrateLegacyContinuation('codex');
+    expect(resultAfterSecondCall).toBeUndefined();
+  });
+
+  test('prefers existing current-provider slot over legacy', () => {
+    seedLegacy('legacy-value');
+    setContinuation('claude', 'claude-value');
+
+    const result = migrateLegacyContinuation('claude');
+
+    expect(result).toBe('claude-value');
+    expect(getContinuation('claude')).toBe('claude-value');
+  });
+
+  test('no legacy row — returns current provider value (possibly undefined)', () => {
+    expect(migrateLegacyContinuation('claude')).toBeUndefined();
+
+    setContinuation('codex', 'codex-value');
+    expect(migrateLegacyContinuation('codex')).toBe('codex-value');
+  });
+
+  test('migration is idempotent on a second call (legacy already gone)', () => {
+    seedLegacy('once');
+
+    const first = migrateLegacyContinuation('claude');
+    expect(first).toBe('once');
+
+    const second = migrateLegacyContinuation('claude');
+    expect(second).toBe('once');
+  });
+});