feat(examples): add key expression argument to z_pub_thr and z_sub_thr

[kydos]

Description

Allow users to specify a custom key expression for the throughput examples via a CLI argument. This makes it possible to benchmark router throughput scaling across multiple instances using either shared or distinct key expressions without modifying the source.

What does this PR do?

See above.

Why is this change needed?

Because otherwise it is not possible to run throughput test on a user specified key-expr.

Related Issues

Closes #2548

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Labels: new feature

🆕 New Feature Requirements

Since this PR adds a new feature:

• Feature scope documented - Clear description of what the feature does and why it's needed
• Minimum necessary code - Implementation is as simple as possible, doesn't overcomplicate the system
• New APIs well-designed - Public APIs are intuitive, consistent with existing APIs
• Comprehensive tests - All functionality is tested (happy path + edge cases + error cases)
• Examples provided - Usage examples in code comments or separate example files
• Documentation added - New docs explaining the feature, its use cases, and API
• Feature flag considered - Can the feature be enabled/disabled for gradual rollout?
• Performance impact assessed - Memory, CPU, storage implications measured
• Integration tested - Feature works with existing features

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[Copilot]

Pull request overview

Adds a CLI argument to the throughput examples so users can choose the key expression at runtime, enabling benchmarking across multiple instances without editing source (addresses #2548).

Changes:

• Add --key-expr (KeyExpr<'static>) argument to z_pub_thr and z_sub_thr (default: test/thr).
• Add --key-expr argument to the shared-memory throughput publisher example (z_pub_shm_thr) and plumb it through parse_args().

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
examples/examples/z_sub_thr.rs	Adds --key-expr and uses it for subscriber declaration; refactors arg parsing.
examples/examples/z_pub_thr.rs	Adds --key-expr and uses it for publisher declaration.
examples/examples/z_pub_shm_thr.rs	Adds --key-expr and passes it through parse_args() to publisher declaration.

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[Copilot]

The error message passed to expect() is misleading here: the failing operation is the subscriber declaration/background start, not opening the Zenoh session (which already happened above). Use an error message that reflects the actual failing call (e.g., declaring/starting the subscriber), or move the expect() to the zenoh::open(...) call.

Suggested change

	.expect("Failed to open Zenoh session");
	.expect("Failed to declare/start Zenoh subscriber");

[fuzzypixelz]

@kydos Copilot is right here, but I would rather write "Failed to declare subscriber".

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.82%. Comparing base (9545f32) to head (7ec87c3).
⚠️ Report is 22 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2549      +/-   ##
==========================================
- Coverage   74.83%   74.82%   -0.02%     
==========================================
  Files         399      399              
  Lines       59373    59373              
==========================================
- Hits        44433    44426       -7     
- Misses      14940    14947       +7     

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