Part of #953

What

Raise line coverage of src/protocols/quic/transport_params.cpp from 4.2% to >= 70%, and branch coverage to >= 60%.

Current state (pre-Step-1 snapshot, 2026-04-13 measurement from parent epic #953):

• Lines hit: 12 / 289 (4.2%)
• File size: 289 LOC

Note: These numbers predate the BUILD_WITH_* workflow fix (#987, #989). A re-measurement on the current develop baseline will be captured in the PR description before merging.

Why

• QUIC transport parameters implement the wire-level negotiation required by RFC 9000 §18; malformed parameter encoding causes handshake failures that are hard to debug post-deploy.
• 4.2% line coverage means parameter encode/decode is almost entirely exercised along a single happy path — varint edge cases, unknown parameter IDs, malformed length prefixes, and boundary values (0, max uint62) are unverified.
• Parameter negotiation bugs are classic interop failures; the test suite is the first defense.

Where

• Source: src/protocols/quic/transport_params.cpp
• Headers: include/kcenon/network/protocols/quic/transport_params.hpp (verify exact path)
• Existing tests to extend: tests/unit/ — check for existing transport_params_test.cpp before adding
• Companion coverage: pair with existing quic_crypto_test.cpp (test: expand coverage for src/protocols/quic/crypto.cpp #993) for handshake-stage test locality

How

Approach

1. Identify every kQUIC_* transport parameter ID and value range (RFC 9000 §18.2).
2. Write parameterized tests covering:
   • Varint encoding: boundary values (0, 63, 64, 16383, 16384, 2^30-1, 2^62-1)
   • Parameter-ID dispatch including unknown-ID handling
   • Length-prefix validation (length mismatches, zero-length, oversized)
   • Round-trip encode→decode symmetry for each parameter
   • Error paths: truncated buffers, trailing garbage, duplicate parameter IDs
3. Consider fuzz-style randomized round-trip tests if the existing test harness supports them.

Acceptance Criteria

• src/protocols/quic/transport_params.cpp line coverage >= 70%
• src/protocols/quic/transport_params.cpp branch coverage >= 60%
• New or extended tests in tests/unit/
• All tests pass on Ubuntu/macOS/Windows CI
• ASAN/TSAN/UBSAN all green on the new tests
• Current develop baseline re-measured in PR description

Dependencies

• Parent: Expand unit test coverage from 40% to 80% #953
• Prerequisite: Step 1 (PR ci(coverage): enable full-ecosystem coverage build #987, ci(coverage): enable messaging_bridge in coverage build #989) — merged.
• Related: test: expand coverage for src/protocols/quic/crypto.cpp #993 (quic/crypto.cpp — closed)