feat: add pagination support to DatabaseSessionService.list_sessions

[stakeswky]

Summary

Resolves google#4621

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

All 88 existing + new tests pass.