test(proto): keep test connections on their initial keys

[n0-grookie]

Description

Every connection starts with a short first key phase — 10..1000 packets, drawn at random in CryptoState::new — so that peers which mishandle key updates fail early. In a test that is not about key updates, that is an unscheduled event with consequences: it arms a KeyDiscard timer shortly afterwards, at a point the test cannot predict. That is what made tests::multipath::open_path_validation_fails_server_side fail intermittently in the daily CI runs, which #797 fixes for that one test.

flub suggested in that review that the harness should not be subject to key updates at all, and matheus23 agreed it should be its own PR. ConnPairBuilder now sets TransportConfig::initial_key_phase_size to a size no test reaches, so a built pair stays on its initial keys, with with_key_update() to opt back in. Pair::seeded() is left alone, so the proptests keep exercising the autonomous update.

API Changes

None, test changes only. The knob is a #[cfg(test)] field on TransportConfig, alongside deterministic_packet_numbers.

Notes & open questions

• Written by n0-grookie, the agent account, from the review thread on test(proto): fix flaky open_path_validation_fails_server_side #797. cargo make green locally.
• On the pre-test(proto): fix flaky open_path_validation_fails_server_side #797 sequence over 3000 seeds: 0 failures with the new default, 9 with with_key_update().
• Whether the older harness (Pair::default() / Pair::new(), ~100 call sites) should get the same default. Those tests build their own TransportConfig, so they keep production behaviour.
• No issue linked: the flake came out of the daily CI run and is discussed on test(proto): fix flaky open_path_validation_fails_server_side #797.

Change checklist

• Self-review.
• Documentation updates following the style guide, if relevant.
• Tests if relevant.
• All API changes documented.
• This PR was created by a human that thought critically about the
  proposed change and wrote an as clear and concise description as
  they could.
• This PR isn't slop, and is carefully crafted to have the intended effect.
• cargo make passes locally.