fix(connectors): prevent state loss on source connector restart

[hubcio]

Source handler tasks were fire-and-forget (JoinHandle dropped),
so the tokio runtime could cancel them mid-state-save during
shutdown. FileStateProvider::save() uses a non-atomic truncate
then write sequence - cancellation between the two left the
state file empty. On restart the connector loaded no tracking
offsets and re-polled all rows from the source database.

Retain handler JoinHandles and await them (with timeout) after
dropping flume senders but before shutting down Iggy clients.
Also add sync_all() after write_all() in save() to ensure
data reaches disk before returning.

[codecov]

Codecov Report

❌ Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.97%. Comparing base (88a598a) to head (a4d2285).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
core/connectors/runtime/src/state.rs	0.00%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2743      +/-   ##
============================================
+ Coverage     68.96%   68.97%   +0.01%     
  Complexity      641      641              
============================================
  Files           736      736              
  Lines         60898    60910      +12     
  Branches      57311    57323      +12     
============================================
+ Hits          41996    42011      +15     
+ Misses        16858    16853       -5     
- Partials       2044     2046       +2     

Flag	Coverage Δ
rust	70.21% <73.33%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
core/connectors/runtime/src/main.rs	86.09% <100.00%> (+0.12%)	⬆️
core/connectors/runtime/src/source.rs	66.29% <100.00%> (+0.56%)	⬆️
core/connectors/runtime/src/state.rs	56.00% <0.00%> (-4.87%)	⬇️

... and 12 files with indirect coverage changes

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