Mark conformance helper crates as non-publishable implementation tooling

[Teakowa]

Goal

Make the Language Provider Protocol repository's Cargo package intent explicit so conformance-only helper crates cannot accidentally become public crates.io contracts.

Context

The Rust workspace currently contains lpp-mock-provider and lpp-conformance-runner. They exist to exercise the process/wire protocol and conformance fixtures, not as reusable protocol library packages. Neither package currently declares publish = false, so Cargo treats them as publishable by default even though the repository has no crates.io release workflow today.

The durable public contract of this repository is the language-provider process/wire protocol and its conformance behavior, not the internal Rust packaging used to test it.

Scope

• Mark conformance-only Cargo packages as non-publishable.
• Apply the same explicit policy to future test/fixture/helper Cargo packages in this repository unless a separately reviewed public Rust package contract is introduced.
• Keep conformance binaries runnable from the workspace and CI exactly as needed for protocol verification.
• Document the distinction only where necessary to keep package intent clear to maintainers.

Non-goals

• Creating a new public Rust SDK or protocol crate.
• Changing LPP wire semantics, fixtures, or conformance behavior.
• Reorganizing the conformance harness for style reasons.
• Adding crates.io release automation.

Acceptance criteria

• lpp-mock-provider and lpp-conformance-runner are explicitly non-publishable Cargo packages.
• Existing workspace builds, tests, and conformance execution remain unchanged.
• No public package/API contract is implied by the Rust helper-crate decomposition.

Ownership

language-provider-protocol owns the process/wire contract and its conformance harness. A future public Rust SDK, if justified, must be introduced as an explicit separately reviewed contract rather than by publishing a conformance helper.