Add tracing span for full snapshot creation duration

[alco]

Summary

• We previously had no tracing span that covered the full duration of initial snapshot building, making it impossible to get historical data on how long snapshots take for any customers
• Added a new shape_snapshot.stream_and_store span that wraps the entire snapshot streaming and storage process, including the DB query execution, data streaming, and writing to storage
• The OTel context is propagated from the Snapshotter process into the spawned task to maintain the correct parent-child span hierarchy

The updated span hierarchy is now:

shape_snapshot.create_snapshot_task
  ├─ shape_snapshot.prepare_tables
  │    ├─ shape_snapshot.start_readonly_txn
  │    ├─ shape_snapshot.get_pg_snapshot
  │    └─ shape_snapshot.set_display_settings
  └─ shape_snapshot.stream_and_store        ← NEW (runs in spawned task)
       └─ shape_read.stream_initial_data

Test plan

• All existing unit tests pass (443 tests, 0 failures)
• Verify the new span appears in traces when creating a shape snapshot

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.22%. Comparing base (03943ad) to head (a083448).
⚠️ Report is 4 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3867      +/-   ##
==========================================
+ Coverage   87.16%   87.22%   +0.06%     
==========================================
  Files          25       25              
  Lines        2314     2325      +11     
  Branches      580      583       +3     
==========================================
+ Hits         2017     2028      +11     
  Misses        295      295              
  Partials        2        2              

Flag	Coverage Δ
packages/experimental	87.73% <ø> (ø)
packages/react-hooks	86.48% <ø> (ø)
packages/start	82.83% <ø> (ø)
packages/typescript-client	92.09% <ø> (+0.05%)	⬆️
packages/y-electric	56.05% <ø> (ø)
typescript	87.22% <ø> (+0.06%)	⬆️
unit-tests	87.22% <ø> (+0.06%)	⬆️

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[blacksmith-sh]

Found 1 test failure on Blacksmith runners:

Failure

Test	View Logs
Elixir.Electric.Replication.PublicationManagerTest/ test component restarts handles relation tracker restart	View Logs