Closes #5366: arkouda.core module

[ajpotts]

Summary

This PR completes the migration of client-facing internals from legacy top-level modules
(arkouda.client, arkouda.logger, arkouda.message, arkouda.infoclass, arkouda.security)
into the new arkouda.core.* namespace.

All internal imports, tests, benchmarks, documentation, and configuration have been updated
to reference arkouda.core as the authoritative implementation, while preserving backward
compatibility via deprecation shims.

---

What changed

Core refactor & API usage

• Replaced imports of:
  • arkouda.client → arkouda.core.client
  • arkouda.logger → arkouda.core.logger
  • arkouda.message → arkouda.core.message
  • arkouda.infoclass → arkouda.core.infoclass
  • arkouda.security → arkouda.core.security
• Updated all uses of:
  • generic_msg
  • _no_op
  • maxTransferBytes
  • array-rank helpers
  • async/wait utilities
    to route through arkouda.core.client.

Deprecation shims

• Added __init__.py shims for:
• arkouda.core.client
• arkouda.infoclass
• arkouda.logger
• arkouda.message
• arkouda.core.security
• Each shim:
  • Emits a DeprecationWarning
  • Re-exports symbols from the corresponding arkouda.core.* module

Tests & benchmarks

• Moved core-related tests under tests/core/
• Updated pytest.ini to include new core test paths and remove deprecated ones
• Updated benchmarks and plotting tests to reference ak.core.client

Documentation & examples

• Updated developer docs, training materials, and examples to import from
  arkouda.core.client
• Ensures new contributors see the correct module structure

Tooling & coverage

• Updated .coveragerc and .flake8 to reflect the new module layout
• Slightly adjusted docstring coverage threshold to account for module moves

---

Why this change

• Establishes a clear separation between public API surface and core implementation
• Enables future removal of deprecated top-level modules without breaking users
• Improves maintainability, test organization, and internal consistency
• Aligns code, tests, docs, and benchmarks on a single source of truth

---

Backward compatibility

• Existing imports from arkouda.client, arkouda.logger, etc. continue to work
• Users receive explicit DeprecationWarnings guiding them to arkouda.core.*
• No functional behavior changes are intended

---

Follow-ups (out of scope)

• Remove deprecated top-level modules in a future major release
• Update public API documentation
• Re-tighten coverage thresholds once deprecations are removed

Closes #5366: arkouda.core module

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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##             main     #5367   +/-   ##
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  Coverage        ?   100.00%           
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  Files           ?         5           
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  Branches        ?         0           
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  Hits            ?       115           
  Misses          ?         0           
  Partials        ?         0           

Flag	Coverage Δ
python-coverage	100.00% <100.00%> (?)

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