docs: make the encrypted session quick start self-contained

[rioyu123]

Summary

The encrypted-session page currently installs only the encrypt extra, but its quick start immediately uses SQLAlchemySession with the aiosqlite driver, so following the page from a clean environment requires dependencies that were never installed.

This pull request makes the quick start self-contained by using the built-in SQLiteSession, which works with the documented encrypt installation. The example also closes the underlying session in a finally block so cleanup still happens if wrapper construction or the agent run fails.

The PostgreSQL example remains available for users who want SQLAlchemy-backed storage, with its required extras and engine cleanup made explicit. The cleanup instruction uses the public engine property, not private _engine state.

SDK APIs and encryption behavior are unchanged.

Test plan

• Verified the quick-start imports and encrypted SQLite round trip in an isolated SDK 0.22.0 environment with only the documented encrypt extra.
• Executed the quick-start body with Runner.run replaced by an offline storage operation. Verified real SQLite cleanup on normal completion, a simulated run failure, and an empty-key failure from the real EncryptedSession constructor. No model API calls were made.
• Verified the PostgreSQL example's dependency and engine-lifecycle instructions against the current SQLAlchemy session API; no live PostgreSQL server was used.
• Passed uv run docs/scripts/generate_ref_files.py and uv run mkdocs build, the two commands in make build-docs, on Windows where make is unavailable. Verified the revised English page in the generated output.
• Passed git diff --check and completed independent review of the final documentation content.

Issue number

N/A

Checks

• I've added new tests, if relevant (documentation-only change; validated with focused offline probes).
• I've run .agents/skills/code-change-verification/scripts/run.sh (not required for this documentation-only content change).
• I've confirmed the applicable documentation verification steps pass.
• If using Codex, I've run /review before submitting this PR (independent review was completed; the /review command was not run).

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-08T01:54:17.483141Z	ba5e570	PR opened
🔒 Security Review	✅ Completed 2026-09-08T01:54:35.653391Z	ba5e570	PR opened

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review" or "@codex security review".

Codex reacts with 👀 while any review is running, comments if it has suggestions, and reacts with 👍 once all reviews finish with no findings.