fix: report the events the SQLite migration actually migrated - #7299

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: SQLite migration reports the source event count after skipping rows #7298

Testing Plan

Problem:

migrate_from_sqlalchemy_sqlite skips any event row whose to_event() raises, warns per row, and then logs Migrated {len(events)} events. — the source row count. The transaction commits and the run ends with Migration completed successfully., so a run that dropped events claims the full count, and the skipped rows are only visible one warning at a time. migrate_from_sqlalchemy_pickle in the same package already reports the number of successful inserts (num_rows).

Solution:

Count the successful inserts and report that, then name the skipped ids in one warning so the operator knows the destination is incomplete while the source database still holds those events.

# before
WARNING  Failed to migrate event event2: unreadable event payload
INFO     Migrated 2 events.            # 1 actually migrated
INFO     Migration completed successfully.

# after
WARNING  Failed to migrate event event2: unreadable event payload
INFO     Migrated 1 events.
WARNING  Skipped 1 event(s) that could not be migrated: event2. They are
         still in the source database; re-run the migration once the cause
         is fixed.
INFO     Migration completed successfully.

Unit Tests:

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

tests/unittests/sessions/migration/test_migration.py::test_sqlite_migration_reports_only_migrated_events builds a v0 database with two events, makes one to_event() raise, runs migrate(), and asserts the destination holds event1 only, the log says Migrated 1 events., does not say Migrated 2 events., and names event2.

• On main (044a1ec) that test fails: the log says Migrated 2 events. with one row in the destination.
• On this branch: .venv/bin/python -m pytest tests/unittests/sessions/migration/ -q → 61 passed.
• pre-commit run --files <changed files> → fix end of files, trim trailing whitespace, ruff, isort, pyink, addlicense, codespell, ADK compliance checks and the doc-link check all pass.
• .venv/bin/python -m mypy src/google/adk/sessions/migration/migrate_from_sqlalchemy_sqlite.py → no issues.

Manual End-to-End (E2E) Tests:

Not run — this is an offline migration script, and the regression test drives the real migrate() against a real v0 SQLite database plus a real destination database, so the only mocked part is the one to_event() call that raises.

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.
• 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 manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

Scope is deliberately one file plus one test: the count, and the warning that names what was dropped. I did not change whether the migration commits when rows are skipped — the pickle path commits too, so that stays a separate decision for maintainers. Happy to drop the second warning if you would rather have only the corrected count.