Remove unconsumed wright-lpp provider-registry surface

[Teakowa]

Summary

Entropy-audit Risk A cut: four wright-lpp registry APIs had no production, embedding, or external consumers. This removes them end to end.

• ProviderRegistry::from_env + LPP_MOCK_PROVIDER_ENV — zero callers anywhere in the workspace. Its doc comment called it "the integration-test/CI hook", but every test suite (wright-lpp/tests/mock_provider.rs, wright-driver/tests/{lpp,provider_edit}.rs) and the CI job read the LPP_MOCK_PROVIDER variable via string literal and register the mock provider explicitly through register(). The env-populated registry was dead code advertising a configuration path nothing exercises.
• ProviderRegistry::get — zero external callers; spawn resolves through the internal map directly.
• ProviderRegistry::languages — the only reference was the crate's own test asserting the registered keys; removed with that assertion. The registry-lifecycle protections that matter (duplicate refusal with RegistryError::DuplicateLanguage, spawn success, explicit NotConfigured refusal with no fallback) are untouched.
• ProviderConfig::with_request_timeout — the only caller set the same 30 s value ProviderConfig::new already defaults to, i.e. a no-op. The pub request_timeout field remains directly settable for embedders needing a non-default timeout.

SessionConfig.providers' doc comment now describes the LPP_MOCK_PROVIDER variable accurately as a test/CI mechanism instead of pointing at the removed registry API.

Evidence

• Consumers verified by workspace-wide search before the cut (from_env, get, languages, with_request_timeout, LPP_MOCK_PROVIDER_ENV: zero references outside their own definitions and the two self-test usages removed here).
• wright-lpp is an internal workspace crate (nothing is published to crates.io), and docs/embedding.md does not list any of these APIs in the stable embedding contract — the documented surface (ProviderRegistry, ProviderConfig, register/spawn flow) is unchanged.
• After the cut: grep for all four names returns zero hits.

Validation

Run on the branch with an isolated CARGO_TARGET_DIR (the shared local target/ is concurrently contaminated by another worktree's binaries; GitHub CI has no such sharing):

• cargo fmt --all -- --check — clean
• cargo clippy --locked --workspace --all-targets --all-features -- -D warnings — clean
• cargo test --locked --workspace --all-targets --all-features — 591 passed / 0 failed
• The mock-provider-dependent suites self-skip locally; CI's LPP client integration (#142, #139) job runs them against the pinned lpp-mock-provider build.

Testing-policy notes

• No expectation changed: the two removed test usages were a redundant key-listing assertion and a no-op builder call; every surviving observable-contract assertion is unmodified.
• The surviving registry contract (explicit registration, duplicate refusal, no-fallback refusal) keeps both positive and negative coverage.