refactor(core): compile the mock provider only with a testing feature

Phase 3a of docs/DESIGN.md. MockProvider moves to api/mock.rs, and it,
the embedded MSN fixture bodies, and the IDX_MOCK_* hooks compile only
with idx-core's new `testing` feature. idx-cli enables it as a
dev-dependency, so `cargo test` keeps the fixture-backed provider while
release binaries no longer embed fixture data or honor
IDX_USE_MOCK_PROVIDER. Parser helpers used only by the mock are gated
the same way.

The packaged-cli flake check no longer relies on the mock; it checks
an offline cache miss (JSON CACHEMISS, exit 3) instead.

Goldens unchanged; clippy is clean with and without `testing`.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/idx-core/Cargo.toml

@@ -17,6 +17,8 @@ default = ["ownership"]
 ownership = ["dep:rusqlite", "dep:quick-xml", "dep:sha2", "dep:zip"]
 # Derives clap::ValueEnum on enums frontends expose as CLI arguments.
 clap = ["dep:clap"]
+# Fixture-backed mock provider (IDX_USE_MOCK_PROVIDER) for tests only.
+testing = []
 
 [dependencies]
 clap = { workspace = true, optional = true }