fix(migrations): keep still-live dev intervals in v0071

[anxkhn]

The v0071 intervals migration drops every still-live dev interval on upgrade
because of a type mismatch in a set-membership check.

In `_migrate_intervals`, `used_dev_versions` is a `Set[Tuple[str, str]]` and its
only insertion adds a `(name, dev_version)` 2-tuple:

```python
used_dev_versions.add((name, dev_version))

But the guard that decides whether to drop a dev interval compares a bare
dev_version string against that set:

dev_version = snapshot_ids_to_dev_versions.get((name, identifier))
if dev_version not in used_dev_versions and is_dev:
    # If the interval's dev version is no longer used and this is a dev interval, we can safely delete it
    continue

A str is never an element of a set of 2-tuples, so dev_version not in used_dev_versions is always True and the condition collapses to
if is_dev: continue. Every dev interval that had already passed the preceding
used_versions check (i.e. is still live) is discarded, forcing a re-backfill of
dev environments after the upgrade.

This changes the check to compare the correctly-typed (name, dev_version) key,
matching the tuples stored in used_dev_versions, so only genuinely unused dev
intervals are removed.

Test Plan

• Added test_migrate_v0071_keeps_still_live_dev_intervals in
  tests/core/state_sync/test_state_sync.py. It builds an in-memory DuckDB state
  at schema v70 (intervals table without the dev_version column) with a live
  prod snapshot/interval and a live dev snapshot/interval, runs the v0071
  migrate_schemas + migrate_rows, and asserts both intervals survive.
• Confirmed the test fails without the fix (the dev interval is dropped:
  {'prod_int'} != {'dev_int', 'prod_int'}) and passes with it.
• Also confirmed a genuinely-stale dev interval (its dev version no longer mapped
  to a live snapshot) is still correctly removed, so the fix is not over-broad.
• make style (ruff + ruff-format + mypy) passes on the changed files; the
  tests/core/state_sync/test_state_sync.py migration/interval subset passes.

Checklist

• I have run make style and fixed any issues
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test)
• My commits are signed off (git commit -s) per the DCO