Closes #1032

What

Add tests/unit/tcp_socket_extra_coverage_test.cpp plus CHANGELOG entries
and a CMake registration. The new suite complements the existing
tcp_socket_test.cpp to push coverage for src/tcp_socket.cpp past the
>= 70% line / >= 60% branch bar set by Issue #1032.

Why

src/tcp_socket.cpp is one of the lowest-coverage files in the
network_system source tree per the #953 audit. The uncovered region
concentrates in error and boundary paths (rejection branches, idempotence,
post-close handler delivery, backpressure transitions) — exactly the surfaces
that regress most easily during the ongoing Result<T> migration. Bringing
this file over the 70/60 bar is a direct contribution to the #953 ecosystem-wide
coverage target.

Where

• tests/unit/tcp_socket_extra_coverage_test.cpp (new, +540 lines)
• tests/CMakeLists.txt (+5 lines, registers network_tcp_socket_extra_coverage_test)
• CHANGELOG.md (+1 line)
• docs/CHANGELOG.md (+5 lines)

No source changes under src/tcp_socket.cpp — tests are hermetic and only
exercise the existing public/internal surface.

How

Tests use a dedicated asio::io_context running on a background thread
and a loopback acceptor on a kernel-assigned port (mirrors the existing
tcp_socket_test.cpp fixture). Each test connects a client/server
tcp_socket pair via make_connected_pair() and exercises one of the
following gap areas:

• try_send rejection when max_pending_bytes would be exceeded
• async_send and try_send on a closed socket (handler error path)
• start_read idempotence (compare_exchange_strong false branch)
• start_read after close (early no-op via posted lambda)
• reset_metrics clearing every counter (incl. peak_pending_bytes,
  rejected_sends, backpressure_events)
• Backpressure activation at high_water_mark / release at low_water_mark
• config() accessor for both custom and default configurations
• Multiple observers attached to the same socket all receive data
• detach_observer for an observer that was never attached is a no-op
• Null receive/view/error/backpressure callback setters do not crash on
  later invocation
• stop_read before start_read is harmless and the socket remains usable
• Freshly-constructed default-state invariants (is_closed, pending bytes,
  metrics)

Test Plan

• Local C++ toolchain is unavailable in this sandbox (no cmake / ninja),
  so build and test verification is delegated to CI on this PR.
• Once CI is green, coverage will be reported via the existing Coverage
  Analysis workflow in line with Expand unit test coverage from 40% to 80% #953 / test: expand coverage for src/tcp_socket.cpp #1032 acceptance criteria.

Notes

• Surgical change: tests only. No modification of src/tcp_socket.cpp,
  no refactor, no style normalization.
• Follows the same *_extra_coverage_test.cpp naming and registration
  pattern established by test: expand coverage for src/protocols/http2/hpack.cpp #1031 (hpack_extra_coverage_test.cpp).
• All commits, this PR title/body, and the planned issue comment are in
  English per project policy.