Revert blocking state DB initialization

[pakrym-oai]

Why

PR #20689 made app-server startup require successful SQLite state DB initialization. That turns a local state-store failure into a process startup failure. Before that change, Codex logged the initialization failure and continued with DB-backed features disabled or degraded.

This PR restores the previous nonblocking behavior without creating a temporary or fake runtime DB.

What Changed

• Treat the process state DB as Option<StateDbHandle> through app-server, in-process app-server, core ThreadManager, MCP server, and local ThreadStore.
• Start log DB and remote control only when the real state DB initializes; return explicit unavailable errors for DB-only operations such as device keys, thread goals, and memory reset.
• Let rollout-backed thread reads/listing/name updates continue without SQLite, while preserving DB-backed metadata/search when a real DB exists.
• Update tests to create a real per-test state DB only for scenarios that exercise DB-backed behavior.

Verification

• cargo test -p codex-app-server --lib
• cargo test -p codex-core --lib
• cargo test -p codex-thread-store
• targeted codex-core agent/session/multi-agent tests
• cargo check -p codex-core -p codex-app-server -p codex-mcp-server -p codex-thread-manager-sample -p codex-thread-store -p codex-app-server-client
• just fix -p codex-core -p codex-core-api -p codex-app-server -p codex-app-server-client -p codex-mcp-server -p codex-thread-manager-sample -p codex-thread-store