fix(session): bound PocketBase latest-message reads - #2042

Description

Keep PocketBase latest-message reads bounded when regenerating a response.

The regenerate path calls get_last_message twice: once for the latest user message and once for the conversation tail. The store used get_full_list with perPage: 1, but the Python SDK replaces that parameter with its batch size and follows every page. Long conversations therefore downloaded their matching history twice just to select two records.

Use get_list(1, 1) and keep the existing session/role filters, descending timestamp order, serialization, and empty/error results. Add regression coverage for filtered and unfiltered reads, no matching role, and an inaccessible session.

Related Issues

Follow-up to the regenerate feature in #349. This does not implement #2030's separate lazy-loading proposal.

Module(s) Affected

• services
• tests

Validation

• 84 tests passed across the PocketBase fallback/isolation, workspace-scope, and regenerate suites. The three bounded-query cases fail on the unchanged base and pass with this fix.
• pre-commit run --all-files: all hooks passed, including Ruff, Prettier, secret detection, Bandit, and MyPy.
• ruff check . and ruff format --check .: passed with CI-pinned Ruff 0.16.0.
• Real PocketBase Python SDK 0.17.3 with a local HTTP replay, 320 stored messages and 2 KiB message bodies: the two message-collection lookups went from 6 requests / 480 returned rows to 2 requests / 2 returned rows, selecting the same records. Session-access checks were stubbed in this probe; ownership remains covered by the project tests. This measures SDK pagination, not live-server latency.

Verified on macOS arm64 / Python 3.12.14. The full application/frontend suites, alternate platforms, and a live PocketBase/UI session were not run locally.

Checklist

• This PR targets dev.
• I have read the contribution guidelines.
• Relevant regression tests and complete pre-commit checks pass.
• No public API, schema, dependency, or configuration changes are needed.