feat(connectors): restart connector with new config without runtime restart

[seokjin0414]

Summary

Closes #2417.

• Add POST /sinks/{key}/restart and POST /sources/{key}/restart API endpoints
• Implement stop_connector(), start_connector(), restart_connector() on SinkManager and SourceManager
• Add tokio::sync::watch shutdown channel to sink consume loop for graceful cancellation
• Store Arc<Container> and task handles in Manager for lifecycle control
• Extend RuntimeContext with iggy_clients and state_path for restart resource access
• Replace direct shutdown in main.rs with Manager-based stop_connector() calls

Design

• Sink restart: shutdown signal via watch channel → await task handles (5s timeout) → FFI iggy_sink_close → FFI iggy_sink_open with new config → spawn new consume tasks
• Source restart: cleanup flume sender → await forwarding tasks (5s timeout) → FFI iggy_source_close → load state → FFI iggy_source_open with new config → spawn new handler
• Container (shared library) is NOT reloaded — only closed/reopened with new config and plugin ID

[codecov]

Codecov Report

❌ Patch coverage is 81.50745% with 211 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.89%. Comparing base (7ebce1b) to head (48ae540).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
core/connectors/runtime/src/manager/source.rs	72.75%	89 Missing and 2 partials ⚠️
core/connectors/runtime/src/source.rs	74.88%	43 Missing and 10 partials ⚠️
core/connectors/runtime/src/manager/sink.rs	94.04%	10 Missing and 10 partials ⚠️
core/connectors/runtime/src/sink.rs	87.05%	11 Missing and 7 partials ⚠️
core/connectors/runtime/src/api/source.rs	5.55%	17 Missing ⚠️
core/connectors/runtime/src/main.rs	85.33%	8 Missing and 3 partials ⚠️
core/connectors/runtime/src/api/sink.rs	94.11%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2781      +/-   ##
============================================
+ Coverage     67.65%   67.89%   +0.24%     
  Complexity      708      708              
============================================
  Files          1030     1030              
  Lines         83846    84700     +854     
  Branches      60704    61558     +854     
============================================
+ Hits          56722    57509     +787     
- Misses        24771    24823      +52     
- Partials       2353     2368      +15     

Flag	Coverage Δ
rust	70.01% <81.50%> (+0.31%)	⬆️

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/context.rs	89.47% <100.00%> (+1.37%)	⬆️
core/connectors/runtime/src/api/sink.rs	56.14% <94.11%> (+10.26%)	⬆️
core/connectors/runtime/src/main.rs	84.46% <85.33%> (-1.64%)	⬇️
core/connectors/runtime/src/api/source.rs	42.10% <5.55%> (-3.83%)	⬇️
core/connectors/runtime/src/sink.rs	74.34% <87.05%> (+5.62%)	⬆️
core/connectors/runtime/src/manager/sink.rs	94.11% <94.04%> (+30.48%)	⬆️
core/connectors/runtime/src/source.rs	69.49% <74.88%> (+3.19%)	⬆️
core/connectors/runtime/src/manager/source.rs	75.83% <72.75%> (+10.38%)	⬆️

... and 13 files with indirect coverage changes

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[mmodzelewski]

@seokjin0414 Thanks for the PR. I left 2 comments. As the logic for sinks/sources is similar, they apply to both equally.
Also, would you be able to write an integration test under core/integration/connectors, at least one case where we can see that the connector is using the updated config? I think with the new test harness, it should be straightforward.

[seokjin0414]

@mmodzelewski
Thanks for the thorough review!
Added restart integration test using Postgres sink fixture in core/integration/tests/connectors/postgres/restart.rs
It verifies:
send messages → confirm in DB → restart via API → send more messages → confirm all in DB
(2ddb9f9)