Remove unused client message helpers from public API (#202)

[octoaide]

Summary

Remove the public re-exports of the send_err, send_ok, and
send_request helpers from the client module. These symbols were
re-exported from oinq::message but are no longer used by clients,
so keeping them public only widened the crate's public API surface.

What changed

• src/client.rs: removed pub use oinq::message::{send_err, send_ok, s send_request}; so the helpers are no longer part of the public API.
• CHANGELOG.md: documented the removal under [Unreleased] → Removed and
  pointed users to the typed client::Connection APIs instead.

Why

Clients don't use these helpers anymore. Keeping unused re-exports in
the public API makes the crate harder to maintain and increases
backwards-compatibility burden. Internal code that needs the helpers
continues to reference oinq::message directly.

Verification

• cargo test --all-features: all tests passed
• cargo clippy --tests --all-features: clean

Files changed

• src/client.rs
• CHANGELOG.md

Closes #202

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.61%. Comparing base (611db35) to head (5f211b8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #203   +/-   ##
=======================================
  Coverage   85.61%   85.61%           
=======================================
  Files          15       15           
  Lines        6481     6481           
=======================================
  Hits         5549     5549           
  Misses        932      932           

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