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 83.76%. Comparing base (3cd0860) to head (f715f3c).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #3927   +/-   ##
=========================================
  Coverage     83.76%   83.76%           
  Complexity     1358     1358           
=========================================
  Files          1212     1213    +1     
  Lines        166189   166215   +26     
  Branches     133663   133663           
=========================================
+ Hits         139210   139236   +26     
  Misses        23342    23342           
  Partials       3637     3637           

Components	Coverage Δ
Rust Core	84.55% <ø> (ø)
Java SDK	66.67% <ø> (ø)
C# SDK	76.08% <ø> (ø)
Python SDK	90.11% <100.00%> (+0.11%)	⬆️
PHP SDK	84.48% <ø> (ø)
Node SDK	95.84% <ø> (ø)
Go SDK	68.32% <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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