refactor(server): improve server command handlers

[hubcio]

Handlers had duplicated validation and resolution logic scattered across bypass_auth variants and inline code. Entity writes occurred on any shard, requiring distributed filesystem locks that added complexity and potential for race conditions.

Centralizing execution in shard/execution.rs with pre-resolved entity types (ResolvedStream, ResolvedTopic, etc.) eliminates duplicate code paths and ensures entity file writes happen exclusively on shard0, removing the need for fs_locks entirely.

[codecov]

Codecov Report

❌ Patch coverage is 84.86118% with 289 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.00%. Comparing base (0b4cc90) to head (af6737c).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
core/server/src/shard/execution.rs	85.39%	9 Missing and 69 partials ⚠️
core/server/src/metadata/reader.rs	90.41%	23 Missing and 23 partials ⚠️
core/server/src/shard/system/utils.rs	79.54%	24 Missing and 3 partials ⚠️
core/server/src/shard/system/messages.rs	61.76%	18 Missing and 8 partials ⚠️
core/server/src/shard/communication.rs	48.88%	20 Missing and 3 partials ⚠️
core/server/src/http/http_shard_wrapper.rs	50.00%	18 Missing and 3 partials ⚠️
core/server/src/shard/system/consumer_offsets.rs	73.97%	18 Missing and 1 partial ⚠️
core/server/src/shard/handlers.rs	81.91%	0 Missing and 17 partials ⚠️
...er/src/binary/handlers/system/get_stats_handler.rs	50.00%	2 Missing and 2 partials ⚠️
core/server/src/http/consumer_offsets.rs	73.33%	0 Missing and 4 partials ⚠️
... and 14 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2669      +/-   ##
==========================================
+ Coverage   68.85%   69.00%   +0.15%     
==========================================
  Files         575      575              
  Lines       55672    55555     -117     
  Branches    55672    55555     -117     
==========================================
+ Hits        38334    38338       +4     
+ Misses      15467    15338     -129     
- Partials     1871     1879       +8     

Flag	Coverage Δ
rust	69.00% <84.86%> (+0.15%)	⬆️

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

Files with missing lines	Coverage Δ
core/binary_protocol/src/utils/mapper.rs	79.29% <100.00%> (+0.09%)	⬆️
...s/consumer_groups/create_consumer_group_handler.rs	89.47% <ø> (+26.14%)	⬆️
...s/consumer_groups/delete_consumer_group_handler.rs	89.47% <ø> (+28.75%)	⬆️
...ers/consumer_groups/join_consumer_group_handler.rs	89.47% <ø> (+12.20%)	⬆️
...rs/consumer_groups/leave_consumer_group_handler.rs	89.47% <ø> (+12.20%)	⬆️
.../consumer_offsets/store_consumer_offset_handler.rs	84.61% <100.00%> (+2.56%)	⬆️
.../handlers/messages/flush_unsaved_buffer_handler.rs	80.00% <ø> (ø)
.../binary/handlers/messages/poll_messages_handler.rs	93.10% <100.00%> (-0.75%)	⬇️
.../binary/handlers/messages/send_messages_handler.rs	100.00% <ø> (ø)
...y/handlers/partitions/create_partitions_handler.rs	89.47% <ø> (+12.20%)	⬆️
... and 58 more

... and 11 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.