feat: #3017 add ValkeySession provider for session memory

[daric93]

Closes #3017

Summary

Adds ValkeySession, a session memory backend using the Valkey-GLIDE client library. Follows the same patterns as RedisSession — implements the Session protocol, supports TTL, from_url factory, client ownership tracking, and pipelined writes via Batch.

Changes

• Implementation (valkey_session.py): ValkeySession class with get_items, add_items, pop_item, clear_session, ping, close, from_url. Uses Batch for pipelined writes (on par with Redis's pipeline). Supports valkey://, valkeys://, redis://, rediss:// URL schemes and ServerCredentials for auth.
• Registration (init.py): Lazy import with pip install openai-agents[valkey] error message.
• Dependency (pyproject.toml): valkey = ["valkey-glide>=2.1"] optional extra, mypy override for glide.
• Unit tests (test_valkey_session.py): 29 tests with lightweight in-memory stub — CRUD, limits, TTL, isolation, ownership, corrupted data, URL parsing, Runner integration.
• Integration tests (test_valkey_integration.py): 29 tests against a real Valkey server via testcontainers (DockerContainer + wait_for_logs, same pattern as test_dapr_redis_integration.py). Skips when Docker is unavailable.
• Docs (index.md, valkey_session.md, mkdocs.yml): Added to comparison table, dedicated section with examples, API reference stub, nav entry.

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 69e4857431

ℹ️ About Codex in GitHub

Codex has been enabled to automatically 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".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

When you sign up for Codex through ChatGPT, Codex can also answer questions or update the PR, like "@codex address that feedback".

[seratch]

As I mentioned at #3017 (comment), we don't plan to add this in the short term, but we may revisit it if many developers need it.