fix(sessions): Reject negative recent-event limits - #6687

Link to Issue or Description of Change

No existing issue or pull request covers this case. I searched open and closed issues and pull requests for num_recent_events, GetSessionConfig, negative limits, and validation before submitting.

Problem:

GetSessionConfig.num_recent_events documents only None, zero, and positive values, but currently accepts negative integers. Those values behave inconsistently across session backends: Python list slicing can drop the oldest event (for example, events[-(-1):] becomes events[1:]), while SQL backends can interpret LIMIT -1 as no limit and return every event.

Steps to reproduce:

1. Construct GetSessionConfig(num_recent_events=-1).
2. Pass it to different session service implementations.
3. Observe that configuration succeeds and backend-specific filtering produces different results.

Expected behavior:

Unsupported negative limits should fail at configuration time, before a session backend handles them.

Observed behavior:

Negative limits are accepted and can either omit events or return all events depending on the backend.

Solution:

Add a Pydantic ge=0 field constraint to the public configuration model. This is the smallest common validation boundary and keeps every backend consistent without duplicating checks.

Environment:

• Source revision: dd0de522
• Desktop OS: macOS
• Python: 3.12.13
• Model information: N/A (session configuration validation does not invoke a model)
• Frequency: Always

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Results:

• Regression test before the fix: 1 failed (DID NOT RAISE ValueError)
• Regression test after the fix: 1 passed
• tests/unittests/sessions/test_session_service.py: 166 passed
• tests/unittests/sessions: 311 passed
• Changed-file pre-commit checks: ruff, isort, pyink, addlicense, and codespell passed. Two repository-level sparse-checkout hooks could not run because their scripts were not present locally; no hook reported a changed-file failure.

Manual End-to-End (E2E) Tests:

N/A. The change is deterministic validation on a public Pydantic configuration model and is covered directly by an isolated unit test; no external service or model is involved.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas (no additional comment was needed for the declarative constraint).
• I have added tests that prove my fix is effective or that my feature works.
• New and existing relevant unit tests pass locally with my changes.
• I have manually verified the configuration behavior before and after the fix.
• No dependent downstream changes are required.

Additional context

AI assistance was used to investigate the cross-backend behavior, search for duplicate issues and pull requests, implement the focused change, and run verification. I reviewed the final diff and test results before submission.