feat(client): make the initialize handshake timeout configurable

[julien-camus-sonarsource]

Why

request() applies the same 30s ceiling to every JSON-RPC round trip, including initialize. That's the right default for steady-state calls against a server that's already running, but it also caps how long the startup command has to get there — and for npx/uvx-based servers, startup can include downloading and installing a package on first run. A consumer with a slower registry or a larger package currently has no way to give that one-time cost more room without also loosening detection of a genuinely hung steady-state call.

Needed by Gitar's custom-integration feature (private npm registries): the 30s budget is comfortably enough for a small server, but times out on a cold cache for a mid-size one, confirmed in both staging and prod.

What changed

• Client::request_with_timeout(method, params, timeout) — request() now delegates to it with the existing 30s default, so no caller of request/list_tools/call_tool sees a behavior change.
• Client::initialize_with_timeout(implementation, capabilities, timeout) — same split for the handshake; initialize() delegates with 30s.
• ClientBuilder::handshake_timeout(Duration) — opt-in override; spawn_and_initialize() uses it when set, otherwise unchanged 30s default.
• The process-liveness poll inside request_with_timeout now scales its iteration count to the given timeout instead of being hardcoded to 100 * 300ms.

Testing

Two new tests in src/client/test.rs:

• test_handshake_timeout_overrides_the_default — spawns sleep 100 (never speaks the protocol) with a 500ms override and asserts failure well under 5s.
• test_request_default_timeout_is_unchanged_by_handshake_timeout — same sleep, plain request(), asserts it's still pending after a 2s probe (i.e. hasn't fallen back to some other short timeout).

The 11 pre-existing notes-simple/uvx tests fail the same way on unmodified c157b15 in this sandbox (network/package availability, not something this PR touches) — confirmed via git stash.