Test gaps in parameterization, serialization round-trips, and getPlatform

[fohara]

While mutation-testing this repo (I'm building a tool in this space), I ran the vitest suites of the 28 hermetic packages against 1,038 mutants. 866 were caught, which is strong. The 169 survivors are heavily clustered, and three files hold about a third of them:

1. packages/client-engine-runtime/src/parameterization/parameterize.ts (27 survivors): swapping &&/|| in the selection guards (e.g. :154, :224) and negating sel.selection (:158) all pass. The parameterization logic has broad line coverage but the tests mostly don't distinguish these branch outcomes.

2. packages/param-graph/src/serialization.ts (17 survivors): serialize and deserialize each have coverage, but mutants that corrupt one direction survive because no test round-trips a value through both and compares.

3. packages/get-platform/src/getPlatform.ts (15 survivors): the platform and libssl detection branches are permissive under mutation. Examples: negating platform === 'freebsd' (:76), flipping the libssl filename guards (:371, :378, :382), and inverting the linux distro check (:507) all pass. On CI runners these branches mostly collapse to one path, so the others go unobserved.

Smaller clusters: generator-helper/src/GeneratorProcess.ts (10) and adapter-mssql/src/connection-string.ts (9).

The round-trip suggestion for serialization.ts is probably the cheapest win: a single property-style test (serialize, deserialize, deep-equal) would kill most of that file's survivors at once.

Scope note: this covers only the hermetic packages. client, migrate, cli, and the engine-integration paths run under separate DB-backed suites that I did not execute, so nothing here speaks to those.

Happy to share the full survivor list or turn the round-trip test into a PR if that's welcome. No action needed otherwise, 83.7% with this operator set is comfortably above most of what I've measured!