feat(seer): send feature attribution for LLM generation

[sehr-m]

Summary

• add a backward-compatible optional feature field to Sentry-to-Seer LLM generation requests
• emit summarization from issue-view title generation

Dependency

Depends on getsentry/seer#8928, which accepts the optional field and establishes it as the ambient feature without overriding a higher-level feature.

Tests

• PYTHONPATH=src .venv/bin/pytest -q --reuse-db tests/sentry/seer/endpoints/test_issue_view_title_generate.py (11 passed)
• changed-file lint and commit/push hooks