refactor(server): use papaya instead of DashMap for shards_table

[krishvishal]

Which issue does this PR close?

Closes #2827

Rationale

shards_table has a read-heavy workload (queried on every message route, written only on partition create/delete/rebalance). papaya's lock-free design is purpose-built for this pattern.

What changed?

• Replace DashMap with papaya::HashMap for the shards_table shared lookup table, which maps partition namespaces to owning shards.
• All call sites updated to use papaya's pin-based guard API: pin() returns a HashMapRef with ergonomic get/insert/remove/iter methods. Only worry is that as long as pin() is not dropped the garbage collector of papaya won't clean memory. This maybe a cause for concern across async boundaries.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.59%. Comparing base (6f01169) to head (ff2dbf2).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2828      +/-   ##
============================================
+ Coverage     67.58%   67.59%   +0.01%     
  Complexity      708      708              
============================================
  Files          1027     1027              
  Lines         83147    83150       +3     
  Branches      60004    60019      +15     
============================================
+ Hits          56195    56208      +13     
+ Misses        24612    24591      -21     
- Partials       2340     2351      +11     

Flag	Coverage Δ
csharp	67.06% <ø> (-0.19%)	⬇️
go	6.84% <ø> (ø)
java	54.02% <ø> (ø)
node	92.26% <ø> (-0.07%)	⬇️
python	0.00% <ø> (ø)
rust	69.64% <100.00%> (+0.02%)	⬆️

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

Files with missing lines	Coverage Δ
core/server/src/main.rs	66.34% <100.00%> (+0.21%)	⬆️
core/server/src/shard/builder.rs	100.00% <100.00%> (ø)
core/server/src/shard/communication.rs	83.47% <100.00%> (+0.13%)	⬆️
core/server/src/shard/mod.rs	90.25% <100.00%> (+7.69%)	⬆️
core/server/src/shard/system/streams.rs	96.52% <100.00%> (ø)
core/server/src/shard/system/topics.rs	90.05% <100.00%> (ø)

... and 9 files with indirect coverage changes

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[numinnex]

Let's not do it for the IggyShard on the server, but rather only in the shard module that we will use in simulator, this way we can profile them both and decide.

[krishvishal]

@numinnex understood. Closing this PR.