fix(session): batch long-term extraction of a large commit without an auto-commit policy

[OnePerson2020]

Description

Long-term extraction is batched only when the session has an auto-commit policy, so a commit without one (explicit commit(), imports/backfills) sends the whole archive to the model in one call however large it is. Auto-commit never lets an archive exceed pending_token_threshold (default 150k), so this is the one unbounded path. A policy-less archive larger than DEFAULT_PENDING_TOKEN_THRESHOLD is now split with that threshold, exactly as an auto-committed archive would be; smaller ones keep the single-call path.

Human Involvement

• A human participated in the implementation or review loop
• This PR was generated entirely by AI agents without human participation in the loop

Related Issue

Fixes #6085

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test update

Changes Made

• extraction_batch.py: long_term_extraction_batch_limits(limits, messages) returns the policy limits, or ExtractionBatchLimits(max_message_tokens=DEFAULT_PENDING_TOKEN_THRESHOLD) for a policy-less archive above that threshold.
• session.py: the long-term step uses it (one call site).
• New test test_long_term_extraction_without_policy_batches_only_a_large_commit.

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this on the following platforms:
  • Linux
  • macOS
  • Windows

echo '{}' > /tmp/min.conf
OPENVIKING_CONFIG_FILE=/tmp/min.conf uv run --extra test pytest tests/unit/session tests/session tests/unit/service/test_session_auto_commit.py -q --no-cov --ignore=tests/session/test_session_commit_race.py
uv run --extra dev ruff check openviking/session/session.py openviking/session/extraction_batch.py tests/unit/session/test_extraction_batch.py
uv run --extra dev ruff format --check (same files)
git diff --check

• New test passes; the set of other failures in these suites is identical before and after the change in my environment (vectordb-engine / full-config / real-LLM tests; test_session_commit_race.py hangs on main locally, so it is excluded). Ruff and git diff --check clean.
• We run the equivalent change in production (since 2026-10-10).

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Additional Notes

No API or data-format change; no new dependency.

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