Part of #953

What

Raise line coverage of src/protocols/quic/connection.cpp from 29.2% to >= 70%, and branch coverage from 17.5% to >= 60%.

Current state (2026-04-17 measurement, source PR #989):

• Instrumented lines: 716 / ~1584 LOC
• Lines hit: 209 / 716 (29.2%)
• Branches: 175 / 999 (17.5%)
• Existing tests: tests/unit/quic_connection_test.cpp — 529 LOC, 42 GTEST cases

The file is the largest uncovered implementation unit in the project. 42 existing test cases only exercise 29.2% of lines, indicating shallow coverage along the main path with most state-machine branches and error-handling branches unreached.

Why

• connection.cpp is core QUIC transport logic. Regressions here surface as subtle wire-level bugs that are expensive to diagnose in production.
• Largest single gap against the v1.0 80% line / 70% branch targets tracked in Expand unit test coverage from 40% to 80% #953.
• Result<T> migration in this area (per project status ~75-80% complete) needs safety-net tests before the remaining migrations land.

Where

• Source: src/protocols/quic/connection.cpp
• Header: include/kcenon/network/.../connection.h (verify exact path during implementation)
• Existing tests to extend: tests/unit/quic_connection_test.cpp
• Possibly add: tests/unit/quic_connection_state_test.cpp (state transitions), tests/unit/quic_connection_error_test.cpp (error paths)

How

Approach

1. Run gcovr --show-branches or open coverage_html/src/protocols/quic/connection.cpp.gcov.html from PR ci(coverage): enable messaging_bridge in coverage build #989's coverage artifact to pinpoint uncovered blocks.
2. Group uncovered regions into: (a) state-machine transitions, (b) error/timeout paths, (c) frame-type dispatch branches.
3. Add test cases per group, preferring table-driven parameterized tests where possible.

Acceptance Criteria

• src/protocols/quic/connection.cpp line coverage >= 70%
• src/protocols/quic/connection.cpp branch coverage >= 60%
• New tests added to tests/unit/quic_connection_*.cpp (one file or multiple)
• All existing tests still pass on Ubuntu/macOS CI
• ASAN/TSAN/UBSAN all green on the new tests
• No regressions in neighbouring files (other QUIC files within +/- 2pp)

Out of Scope

• Rewriting connection.cpp itself. Implementation changes should be opened as a separate issue.
• Coverage of protocol interop (that belongs in integration_tests/, not this issue).

Dependencies

• Parent: Expand unit test coverage from 40% to 80% #953
• Prerequisite: PR ci(coverage): enable messaging_bridge in coverage build #989 (coverage re-baseline) — merged.