enhance: Check segment existence when FlushSegments and add some key logs (#34438)

[bigsheeper]

Check if the segment exists during FlushSegments and add some key logs in write path.

issue: #34255

pr: #34438

[mergify]

@bigsheeper E2e jenkins job failed, comment /run-cpu-e2e can trigger the job again.

[bigsheeper]

/run-cpu-e2e

[codecov]

Codecov Report

Attention: Patch coverage is 78.57143% with 3 lines in your changes missing coverage. Please review.

Project coverage is 84.42%. Comparing base (014820e) to head (839d3d3).
Report is 5 commits behind head on 2.4.

Additional details and impacted files

@@            Coverage Diff             @@
##              2.4   #34472      +/-   ##
==========================================
+ Coverage   80.78%   84.42%   +3.64%     
==========================================
  Files        1046      786     -260     
  Lines      136101   112422   -23679     
==========================================
- Hits       109945    94914   -15031     
+ Misses      21948    13305    -8643     
+ Partials     4208     4203       -5     

Files	Coverage Δ
internal/datanode/flow_graph_dd_node.go	92.63% <100.00%> (+0.09%)	⬆️
internal/datanode/syncmgr/task.go	100.00% <100.00%> (ø)
pkg/mq/msgdispatcher/dispatcher.go	96.62% <100.00%> (-0.68%)	⬇️
pkg/mq/msgdispatcher/manager.go	94.57% <100.00%> (-2.40%)	⬇️
internal/datanode/writebuffer/write_buffer.go	85.97% <50.00%> (-0.05%)	⬇️

... and 294 files with indirect coverage changes

[mergify]

@bigsheeper E2e jenkins job failed, comment /run-cpu-e2e can trigger the job again.

[czs007]

/approve
/lgtm

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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[czs007]

/run-cpu-e2e