feat(connectors): let sinks control when offsets are committed

[kriti-sc]

Closes #2928, #2927
Related: #3203

Rationale

Sinks must have an option to decide exactly when it is safe to advance the offset, as only the sink knows when a write is truly durable from the target's perspective.

What changed?

This PR adds an offset_commit setting to sink configs:

• after_polling (default) - existing behaviour of offset being committed when messages are polled. Nothing changes for existing configs.
• after_consuming - the offset is stored only once the sink reports that it wrote the batch.

The offset commit decision hinges upon the i32 returned by the sink.

This also required binding the i32 returned by the sink, which was previous discarded. Now, the sink exits if i32 != 0, regardless of offset commit configuration, which is the correct behaviour as a sink returns error for failures lying outside its purview.

This PR also introduces integration tests that inject failures into a running sink and verify the desired commit behaviour.

[github-actions]

Thanks for the PR. It is labeled S-waiting-on-review and queued for review.

Slash commands (own line, regular comment) move it around the queue:

• /ready - back to S-waiting-on-review after addressing feedback
• /author - flip to S-waiting-on-author while you finish changes
• /request-review @user-or-team - request a reviewer

See CONTRIBUTING.md for details.

[codecov]

Codecov Report

❌ Patch coverage is 92.98246% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.76%. Comparing base (cc269ef) to head (3b2a144).

Files with missing lines	Patch %	Lines
core/connectors/runtime/src/sink.rs	85.71%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3954      +/-   ##
============================================
- Coverage     83.85%   83.76%   -0.10%     
  Complexity     1358     1358              
============================================
  Files          1212     1212              
  Lines        166843   166900      +57     
  Branches     134304   134487     +183     
============================================
- Hits         139905   139800     -105     
- Misses        23298    23305       +7     
- Partials       3640     3795     +155     

Components	Coverage Δ
Rust Core	84.60% <92.98%> (-0.03%)	⬇️
Java SDK	66.67% <ø> (ø)
C# SDK	74.92% <ø> (-1.61%)	⬇️
Python SDK	90.13% <ø> (ø)
PHP SDK	84.48% <ø> (ø)
Node SDK	95.94% <ø> (+0.09%)	⬆️
Go SDK	68.37% <ø> (ø)

Files with missing lines	Coverage Δ
core/connectors/runtime/src/configs/connectors.rs	63.57% <100.00%> (+22.50%)	⬆️
core/connectors/runtime/src/error.rs	28.57% <ø> (ø)
core/connectors/runtime/src/main.rs	85.81% <ø> (ø)
core/connectors/runtime/src/manager/sink.rs	94.17% <100.00%> (+0.01%)	⬆️
core/connectors/runtime/src/sink.rs	77.53% <85.71%> (-1.37%)	⬇️

... and 69 files with indirect coverage changes

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