feat(python): support message partitioning strategies

[jiengup]

Which issue does this PR address?

Closes #3896

Rationale

The Python SDK only supported sending messages to an explicit partition, while the Rust SDK also supports balanced and message-key partitioning.

What changed?

IggyClient.send_messages() now accepts a Python Partitioning object supporting fixed, balanced, and message-key routing. Existing integer partition IDs remain fully compatible, while string message keys are encoded as UTF-8 and validated against the 1–255 byte limit.

Type stubs, tests, and a runnable Python example covering all three strategies are included.

Local Execution

• Passed all 87 tests in foreign/python/tests/test_message_operations.py
• Passed Rust formatting and Clippy checks
• Passed Ruff and Pyrefly checks for the Python SDK and examples
• Successfully ran the partitioning example against a local Iggy server
• Pre-commit hooks ran and passed

AI Usage

1. Tool: OpenAI Codex
2. Scope: Repository analysis, PyO3 implementation, tests, type stubs, documentation, and the Python example
3. Verification: Reviewed the generated changes, ran the complete message operations test suite, static checks, pre-commit hooks, and the example against a local server
4. Yes, I can explain every line of the changes

[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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.19%. Comparing base (ffb186d) to head (7b4ec8f).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #3927   +/-   ##
=========================================
  Coverage     84.18%   84.19%           
  Complexity     1398     1398           
=========================================
  Files          1219     1220    +1     
  Lines        173564   173590   +26     
  Branches     140662   140662           
=========================================
+ Hits         146122   146148   +26     
  Misses        23472    23472           
  Partials       3970     3970           

Components	Coverage Δ
Rust Core	84.99% <ø> (ø)
Java SDK	67.23% <ø> (ø)
C# SDK	75.39% <ø> (ø)
Python SDK	90.18% <100.00%> (+0.11%)	⬆️
PHP SDK	85.65% <ø> (ø)
Node SDK	96.07% <ø> (ø)
Go SDK	69.09% <ø> (ø)

Files with missing lines	Coverage Δ
foreign/python/src/client.rs	99.85% <100.00%> (+<0.01%)	⬆️
foreign/python/src/lib.rs	100.00% <100.00%> (ø)
foreign/python/src/partitioning.rs	100.00% <100.00%> (ø)

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[ethanlin01x]

Some suggestions. The stub one I think should be fixed before merge.

[jiengup]

apache_iggy.pyi is re-generated through stub_gen binary and formatted by ruff, some changes are unrelated to this PR.

[jiengup]

/ready