feat(go): add stream/topic purge API with BDD tests and fix server purge bug

[chengxilo]

Which issue does this PR address?

Relates to #2148

Rationale

Go SDK doesn't have the purge topic/ purge stream API, so this PR implement them. To test it, I added an new BDD test scenario, and implemented both Go and Rust BDD tests.

What changed?

The Go SDK was missing PurgeStream and PurgeTopic methods. Added them along with a shared BDD scenario (stream_topic_purge.feature) and implementations in both Go and Rust.

While testing, discovered that purge_all_segments only cleared on-disk segments but left stale messages in the journal and in-flight tiers. Polling after purge would return messages that should have been deleted. Fixed by calling journal.reset() and clear_in_flight() during partition re-initialization. I am not famliar with Rust server code so during the code review this part worth more attention

Local Execution

• Passed
• Pre-commit hooks ran

AI Usage

1. DeepSeek pro v4, Claude Opus 4.6
2. DeepSeek pro v4 for BDD test implementation, Claude Opus 4.6 for server bug fixing.
3. I reviewed them with my best effert, the bdd scripts are executed and actually works, and they found that there is a bug in the server so I believe they works.
4. I hope so.

[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 30.00000% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.47%. Comparing base (6f68d92) to head (eb9c363).

Files with missing lines	Patch %	Lines
foreign/go/internal/command/stream.go	0.00%	4 Missing ⚠️
foreign/go/internal/command/topic.go	0.00%	4 Missing ⚠️
foreign/go/client/tcp/tcp_stream_management.go	0.00%	3 Missing ⚠️
foreign/go/client/tcp/tcp_topic_management.go	0.00%	3 Missing ⚠️

❌ Your patch check has failed because the patch coverage (0.00%) is below the target coverage (50.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3608       +/-   ##
=============================================
- Coverage     73.63%   42.47%   -31.17%     
  Complexity      937      937               
=============================================
  Files          1284     1281        -3     
  Lines        140151   121547    -18604     
  Branches     116041    97464    -18577     
=============================================
- Hits         103204    51625    -51579     
- Misses        33726    67151    +33425     
+ Partials       3221     2771      -450     

Components	Coverage Δ
Rust Core	34.73% <100.00%> (-39.17%)	⬇️
Java SDK	62.44% <ø> (ø)
C# SDK	71.40% <ø> (-0.73%)	⬇️
Python SDK	91.65% <ø> (ø)
PHP SDK	84.29% <ø> (ø)
Node SDK	91.26% <ø> (-0.10%)	⬇️
Go SDK	42.16% <0.00%> (-0.13%)	⬇️

Files with missing lines	Coverage Δ
core/server/src/shard/system/segments.rs	89.94% <100.00%> (+0.54%)	⬆️
core/server/src/streaming/partitions/journal.rs	86.04% <100.00%> (+0.68%)	⬆️
foreign/go/client/tcp/tcp_stream_management.go	76.31% <0.00%> (-6.55%)	⬇️
foreign/go/client/tcp/tcp_topic_management.go	76.78% <0.00%> (-4.35%)	⬇️
foreign/go/internal/command/stream.go	83.78% <0.00%> (-10.16%)	⬇️
foreign/go/internal/command/topic.go	87.17% <0.00%> (-4.72%)	⬇️

... and 395 files with indirect coverage changes

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[hubcio]

@chengxilo i'll check this during the weekend. could you please rebase?

[numinnex]

I've checked whether there is potential concurrency issue due to interleaving of flush + purge commands as this is what I've reckon from the changes could potentially be an issue and did not find any problems with that. Let's wait for @hubcio review and we can merge it.