Make unary_request internal-only (closes #200)

[octoaide]

This PR removes unary_request from the public API and makes it
crate-internal (pub(crate)). The function is no longer intended for
external use and internal callers continue to use crate::unary_request
without changes.

Changes

• src/lib.rs: change unary_request visibility to pub(crate)
• CHANGELOG.md: document removal under [Unreleased] → Removed

Why
Reducing the public API surface clarifies the supported interfaces and
encourages use of the typed helpers (for example, client::Connection and
server::Connection) instead of a raw unary_request entrypoint.

Verification

• cargo test --all-features — 200 tests passed
• cargo clippy --tests --all-features — clean

Migration
If any downstream crate used review_protocol::unary_request directly,
switch to the higher-level APIs (client::Connection, server::Connection,
etc.).

See also: #200

Closes #200

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.68%. Comparing base (b2f4e12) to head (8963ff2).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #201      +/-   ##
==========================================
+ Coverage   85.61%   85.68%   +0.06%     
==========================================
  Files          15       15              
  Lines        6481     6481              
==========================================
+ Hits         5549     5553       +4     
+ Misses        932      928       -4     

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[octoaide]

I reviewed the Codecov report (informational — coverage is unchanged and all modified lines are covered) and investigated the CI failure from the check job.

What I found

• The failing Clippy error was: function unary_request is never used. It happened because unary_request was being compiled any time the server feature was enabled, but in non-test server builds that function is only used by client code and by server tests — so it was legitimately unused in some build combinations.
• Making the function pub(crate) didn’t solve the build combinations problem because it was still being compiled in contexts that don’t call it.

What I changed

• I tightened the conditional compilation for unary_request so it is only compiled when it’s actually needed:
  #[cfg(any(feature = "client", all(test, feature = "server")))]
  This ensures the function is built for the production client API, and also when running server tests (which exercise unary requests), but not for normal server builds that don’t call it.

Verification

• I committed and pushed the change.
• I ran cargo clippy --tests for: no features, client, server, and --all-features — all clean.
• I ran cargo test --all-features — all tests passed.

Result

• The Clippy error is resolved and Codecov remains unchanged (coverage still OK). Everything required by the feedback has been addressed.

Thanks for the pointer — let me know if you want the cfg applied differently or prefer a comment explaining why the gate is narrow.