test: ✅ Add end-to-end completion-handler test for relationship-hop wiring (#219)

[mikebronner]

Summary

Implements #219 — an end-to-end completion-handler regression test for the
relationship-hop wiring that #211 / PR #215 left covered only indirectly.

The unit tests exercise apply_relation_method_hops + columns_for_collection
directly with a hand-built ChainContext, and the diagnostics path is covered
end-to-end through chain_diagnostics. The glue in
main.rs::try_query_chain_completion that dispatches apply_relation_method_hops
inside the real textDocument/completion flow had no regression guard — a
bad re-wire (wrong call order, missing dispatch, lost effective_model) would
have slipped past every existing test. These tests close that gap, mirroring the
diagnostics-side end-to-end fixtures from PR #215.

Changes

• New test file laravel-lsp/src/tests/query_chain_completion_handler.rs,
  registered in laravel-lsp/src/tests/mod.rs. Builds a real
  LaravelLanguageServer via LspService::new(LaravelLanguageServer::new),
  primes initialized_root with a tempdir of User/Competition models and
  database_schema with a seeded provider, then drives the real
  try_query_chain_completion with $user->competitions->where('|').
  • property_receiver_hop_offers_related_table_columns — asserts the
    competitions column type is offered.
  • property_receiver_hop_excludes_parent_only_columns — asserts the
    users-only column email is not offered, proving the hop advanced
    effective_model from User to Competition.
• laravel-lsp/src/database.rs — DatabaseSchemaProvider::set_test_schema
  is now #[doc(hidden)] pub instead of #[cfg(test)]. ⚠️ Reviewer note:
  this is the one production-lib change. Cargo does not enable cfg(test) on a
  library consumed as a dependency, so a #[cfg(test)] seam is invisible to the
  binary's src/tests/ crate — the AC requires priming database_schema from
  there. #[doc(hidden)] keeps it off the documented public surface while making
  it reachable cross-crate. The schema mutation itself is unchanged.

Acceptance Criteria

• New test file laravel-lsp/src/tests/query_chain_completion_handler.rs
  created and registered in laravel-lsp/src/tests/mod.rs.
• Harness builds a real LaravelLanguageServer via
  tower_lsp::LspService::new(LaravelLanguageServer::new), primes
  initialized_root with a tempdir holding User.php (a competitions hasMany
  to Competition::class) and Competition.php, and primes database_schema
  with a DatabaseSchemaProvider whose competitions table carries a column
  (type) absent from users.
• property_receiver_hop_offers_related_table_columns calls
  try_query_chain_completion with /** @var User $user */ and
  $user->competitions->where('|'), cursor inside the where string argument,
  and asserts the competitions column type is among the returned items.
• A companion test asserts a users-only column (email) is not among
  the returned items, confirming the hop advanced effective_model from User
  to Competition.
• Both tests fail when the apply_relation_method_hops dispatch block in
  main.rs is commented out (verified locally: completion returns
  ["id", "email"], so type is missing and email leaks).
• cargo test passes with no new warnings from the test file.

Test Plan

• cargo fmt --check — clean.
• cargo clippy --all-targets — no warnings (CI runs -D warnings).
• cargo test — full unit suite green (lib 1894 + bin 418, including the 2
  new tests).
• Reverse check — neutralized the apply_relation_method_hops dispatch
  block and confirmed both new tests fail, then restored it.
• ℹ️ The integration_tests target needs the CI-only fixture bootstrap
  (cp test-project/.env.example test-project/.env + composer install); those
  pre-existing, environment-dependent tests are untouched by this change and
  pass in CI.

Fixes #219

[mr-sherlock-holmes]

✅ Approved

Review Summary

A tight, well-documented test-only PR that closes the exact gap #219 named: the main.rs::try_query_chain_completion glue dispatching apply_relation_method_hops had no end-to-end regression guard. Reviewed via four blind lens passes (AC conformance, correctness, security, test honesty) over the PR checkout; CI is green (LSP test/fmt/clippy, 3m9s).

Acceptance criteria — all 6 met:

1. ✅ New file tests/query_chain_completion_handler.rs registered in tests/mod.rs:31 (alphabetical).
2. ✅ Real backend via LspService::new(LaravelLanguageServer::new); initialized_root primed with a tempdir of User.php (hasMany competitions) + Competition.php; database_schema seeded so type lives on competitions but not users.
3. ✅ property_receiver_hop_offers_related_table_columns drives try_query_chain_completion with /** @var User $user */ + $user->competitions->where('◊')->get() and asserts type is offered. (Divergence, fine: a trailing ->get() completes the chain shape; cursor is still inside the where string arg.)
4. ✅ Companion property_receiver_hop_excludes_parent_only_columns asserts email (users-only) is absent.
5. ✅ Genuine regression guard, not coverage theatre — confirmed by code logic and your documented reverse check (["id","email"] with the dispatch neutralized). Without the hop, effective_model stays User → users table → type missing (test 1 fails) and email leaks (test 2 fails). The asymmetric schema catches the missing hop from both directions, and the positive assertion rules out a vacuous pass (.expect() on None, empty-vec both fail).
6. ✅ cargo test + clippy -D warnings green in CI.

What's Good

• The asymmetric fixture (type only on competitions, email only on users) is a sharp design — both assertions can pass only if the hop advanced the model. That's a real guard.
• Hermetic and deterministic: set_test_schema seeds the in-memory cache, no live DB or network, tempdir RAII-cleaned.
• You flagged the one production-lib change yourself in the PR body. Concur with the call: the test must live in the binary crate (crate::LaravelLanguageServer), which consumes the lib without cfg(test), so a #[cfg(test)] seam is invisible cross-crate. #[doc(hidden)] pub is the right pragmatic fit here — it mirrors the already-pub invalidate_cache cache-mutator (database.rs:522), and the feature-flag alternative would be strictly worse in this same-package bin-test topology (it'd break plain cargo test in CI). Well-documented rationale on both sides of the seam.

📋 Non-blocking follow-ups

• None.

Ready for @mikebronner to merge.