fix(lsp): preserve opened buffers across config reload

[jo16oh]

Summary

• reload_config was rebuilding source_cache from disk via load_all_sources, wiping the in-memory text of any document the client had opened with didOpen.
• When the file watcher delivered a cvk.json modification event between a didOpen and a subsequent request (rename/diagnostics/etc.), the searcher was rebuilt against stale disk content and missed unsaved edits.
• After loading sources from disk, re-apply opened_documents on top of the cache before rebuilding parse_cache and searcher — same invariant that update_source_cache_from_disk already enforces via its is_open skip.

Background

Surfaced as flakiness in rename_vue_defined_variable (in tests/lsp_html_like.rs). The test:

1. Copies fixtures to a tempdir, then fs::writes cvk.json with lookupFiles.
2. Spawns the LSP — which starts a notify-based recursive watcher.
3. didOpens Component.vue with custom buffer text referencing --vue-color.
4. Requests rename of --vue-color from Tokens.vue.

Under parallel test load, FSEvents would deliver the recent cvk.json write to the watcher mid-test, triggering reload_config() after didOpen had populated the buffer. load_all_sources then re-read Component.vue from disk (original fixture, no --vue-color), and the rename returned no edits for Component.vue.

Single-test or single-threaded runs always passed because the timing window didn't open up.

Test plan

• Run full cargo test 10× — all pass (was previously failing ~40% of the time).
• Run rename_vue_defined_variable in isolation — still passes.
• cargo fmt / cargo clippy clean (enforced by pre-commit hook).

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