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-cli/Cargo.toml

@@ -36,6 +36,8 @@ sha2 = { workspace = true, optional = true }
 ureq = { workspace = true, optional = true }
 
 [dev-dependencies]
+# Tests drive the binary with the mock provider; release builds exclude it.
+idx-core = { workspace = true, features = ["testing"] }
 assert_cmd = "2"
 predicates = "3"
 rusqlite.workspace = true