feat: add pagination support to list_sessions()

[ferponse]

Closes #4871

Summary

DatabaseSessionService.list_sessions() (and all other session service implementations) now support pagination via page_size and page_token parameters.

Changes

base_session_service.py

• Added next_page_token: Optional[str] field to ListSessionsResponse
• Updated list_sessions abstract signature with page_size and page_token kwargs
• Added shared pagination helpers: _resolve_page_size, _encode_page_token, _decode_page_token

database_session_service.py

• Added ORDER BY update_time DESC + LIMIT/OFFSET to the SQL query
• Fetches page_size + 1 rows to detect next page without a separate COUNT query
• Returns next_page_token when more results exist

in_memory_session_service.py

• Sorts sessions by last_update_time descending
• Applies offset/limit slicing with the same pagination contract

sqlite_session_service.py

• Same ORDER BY update_time DESC LIMIT ? OFFSET ? approach as DatabaseSessionService

vertex_ai_session_service.py

• Signature updated for compatibility (pagination not yet wired to the upstream API)

Tests

• 8 new test cases covering: default page size, custom page size, max clamping, full iteration, exact-fit boundary, empty results, backward compatibility, and ordering

Backward Compatibility

All parameters are optional with sensible defaults:

• page_size defaults to 20 (max 100)
• page_token defaults to None (first page)
• Existing callers that pass no pagination args continue to work — they receive the first page of up to 20 sessions with next_page_token=None when there are ≤20 results

Test plan

• Default pagination returns first 20 sessions
• Custom page_size is respected
• page_size > 100 is clamped to 100
• Full iteration via page_token collects all sessions exactly once
• Exact-fit boundary (total == page_size) returns no next token
• Empty result returns no token
• Backward compatible without pagination args
• Results ordered by update_time DESC

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces robust pagination capabilities to the list_sessions() method across all session service implementations. This enhancement allows for more efficient retrieval of session data by enabling clients to request sessions in manageable chunks using page_size and page_token parameters. The changes ensure improved performance and resource utilization, especially for applications handling a large volume of sessions, while maintaining backward compatibility for existing API callers.

Highlights

• Pagination Support: Added pagination via page_size and page_token parameters to DatabaseSessionService.list_sessions() and other session service implementations.
• Base Session Service Updates: Introduced next_page_token to ListSessionsResponse, updated the list_sessions abstract signature, and added shared pagination helper functions (_resolve_page_size, _encode_page_token, _decode_page_token).
• Database Query Enhancements: Modified SQL queries in DatabaseSessionService and SqliteSessionService to include ORDER BY update_time DESC and LIMIT/OFFSET for pagination, fetching an extra row to detect the next page.
• In-Memory Session Service: Implemented sorting by last_update_time descending and applied offset/limit slicing for pagination in the InMemorySessionService.
• Vertex AI Session Service: Updated the list_sessions signature for compatibility, though pagination is not yet wired to the upstream API.
• Comprehensive Testing: Added 8 new test cases covering various pagination scenarios, including default page size, custom page size, max clamping, full iteration, exact-fit boundary, empty results, backward compatibility, and ordering.

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[ferponse]

@sasha-gitg Could you please take a look at this when you get a chance? This is blocking our production deployment — we're hitting severe latency issues with list_sessions() on apps with thousands of sessions (see #4871 and #4621). Would really appreciate a review. Thanks!

[gemini-code-assist]

Code Review

This pull request introduces pagination support to the list_sessions() method across various session service implementations. The changes include adding page_size and page_token parameters, updating the ListSessionsResponse model with a next_page_token, and implementing the pagination logic in DatabaseSessionService, InMemorySessionService, and SqliteSessionService. The implementation correctly uses ORDER BY, LIMIT, and OFFSET (or equivalent slicing) and fetches one extra row to efficiently determine the presence of a next page. The page_token is base64 encoded/decoded for robustness. Comprehensive unit tests have been added to cover various pagination scenarios, ensuring correctness and backward compatibility. Overall, the changes are well-structured and address the pagination requirement effectively.

[gemini-code-assist]

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