test: handler-level LSP harness coverage for on_completion / on_goto_definition

[mikebronner]

Summary

Implements #207 — handler-level LSP harness coverage for the Inertia goto-definition and completion request handlers, the follow-up Holmes flagged in his review of #10 / PR #204.

Prior Inertia coverage stopped at the pure path-math seam (resolve_page_candidates, list_pages in inertia.rs) and never drove the real LSP request handlers that wire those seams to the wire protocol. This adds end-to-end tests that build a real backend through the tower_lsp::LspService harness — the same pattern as flux_goto_def_handler.rs / blade_var_rename_handler.rs — so a regression in the handler arm (a dropped containment guard, an inverted existence check, a phantom or missing target) is now caught.

Changes

• New laravel-lsp/src/tests/inertia_handler.rs, wired into laravel-lsp/src/tests/mod.rs.
• Goto tests drive Backend::resolve_inertia_file on a backend primed with a minimal root-only cached_config.
• Completion tests drive Backend::get_all_inertia_pages on a backend primed with root_path — matching exactly the state each handler reads.
• All fixtures use real files in a tempfile::TempDir; the document cache is left empty so file_exists_cached falls through to real disk probes, as the live server does.

Acceptance Criteria

• A new test file laravel-lsp/src/tests/inertia_handler.rs is created and wired into laravel-lsp/src/tests/mod.rs
• Tests use LspService::new(LaravelLanguageServer::new) to build a real backend — no mock; same harness pattern as flux_goto_def_handler.rs and blade_var_rename_handler.rs
• goto — happy path (.vue): resolve_inertia_file("Dashboard") on a backend primed with a minimal cached_config (root only) and a real resources/js/Pages/Dashboard.vue returns the correct PathBuf
• goto — nested page (.tsx): resolve_inertia_file("Auth/Login") with resources/js/Pages/Auth/Login.tsx returns the nested PathBuf
• goto — unresolvable returns None: resolve_inertia_file("Ghost") with no matching page file returns None
• goto — extension priority: when both Dashboard.vue and Dashboard.tsx exist with no dominant extension primed, .vue wins (first in PAGE_EXTENSIONS order)
• completion — pages listed: get_all_inertia_pages() on a backend primed with root_path pointing at a tempdir containing Dashboard.vue and Auth/Login.tsx returns ["Auth/Login", "Dashboard"] (sorted, without extension)
• completion — empty project: get_all_inertia_pages() with no resources/js/Pages/ directory returns an empty list
• completion — extension filter: non-Inertia files (.php, .blade.php) under resources/js/Pages/ are excluded from the completion list

Test Plan

• All 8 new tests pass (cargo test inertia_handler → 7 test fns covering the 8 AC behaviours, all green)
• Full unit suite green (cargo test lib + bin: 1875 + 395 passed, 0 failed)
• cargo fmt --check clean
• cargo clippy --tests clean (CI gates on -D warnings)
• Test-only change; no production code touched. The 8 integration_tests.rs failures seen locally are pre-existing and environmental — they require the gitignored test-project/ fixture (.env + composer vendor) that CI bootstraps (ci.yml lines 30-59); unaffected by this diff.

Fixes #207

[mr-sherlock-holmes]

✅ Approved

Review Summary

• Pure test-only PR (237 additions, 0 deletions): one new harness file inertia_handler.rs + a one-line tests/mod.rs wiring. No production code touched. CI green (LSP test/fmt/clippy pass).
• All 9 acceptance criteria met — verified each behaviour against the real handler arms, not just the test text:
  • goto_backend primes cached_config — exactly what resolve_inertia_file reads (main.rs:18661 → get_cached_config().root); completion_backend primes root_path — exactly what get_all_inertia_pages reads (main.rs:12467 → list_pages). Each fixture primes the state its handler actually consumes. Honest harness.
  • .vue-wins priority confirmed against PAGE_EXTENSIONS = ["vue","tsx","jsx","svelte"] (inertia.rs:25); with root_path unset, inertia_dominant_extension returns None, so the static order decides — the test's "no dominant primed" premise genuinely holds.
  • Empty-project test drives the real list_pages walk on a real empty tempdir (not the None short-circuit) — meaningful coverage, not a trivial pass.
• Built through LspService::new(LaravelLanguageServer::new), matching the flux_goto_def_handler.rs / blade_var_rename_handler.rs convention. No mocks.
• For the record, two test-honesty observations were considered and judged non-actionable: the .blade.php fixture is excluded via the same ext=="php" branch as .php (redundant-but-correct, faithful to the AC), and the extension-priority test doesn't isolate the static-order path from the dominant tie-break (the AC scoped it to "no dominant primed," which the test honors exactly). Neither is a defect; forcing changes there would be gold-plating beyond the contract.

What's Good

• Tests drive the real Backend handler arms (resolve_inertia_file / get_all_inertia_pages), a genuine step up from the already-covered pure path-math seam in inertia.rs — precisely what #207 asked for.
• Each fixture primes exactly the state its handler reads, and the doc comments explain why (Salsa actor stays inert, document cache left empty so file_exists_cached falls through to live disk probes).
• The unresolvable-page test seeds a real unrelated page first, so the None proves "missing target," not "empty/unconfigured project." Careful work.

📋 Non-blocking follow-ups

• Handler-level coverage for the dominant-extension float branch (main.rs:18673-18679) — when a dominant extension is primed (inertia_default_ext = Some(Some("tsx"))), resolve_inertia_file floats that extension ahead of the static PAGE_EXTENSIONS order. This PR deliberately leaves the dominant unset (the AC scoped it that way), so that production branch has only seam-level coverage in inertia.rs, no handler-level test. A belt-and-suspenders add on #207's own theme. General observation about untouched production code, outside this PR's AC — tracked as a new follow-up issue, not a blocker.

Ready for @mikebronner to merge.