Fix IDE tool response handling

[ThomasK33]

Summary

• Update IDE diagnostic results to return grouped URI-based diagnostic payloads with editor-native ranges and severity names.
• Stop advertising an unsupported resources capability during initialization.
• Harden background file opens by ignoring empty text anchors, preserving focus, returning metadata from the target buffer, and serializing tool JSON compactly.

Validation

• mise exec -- stylua --check lua/claudecode/server/init.lua lua/claudecode/tools/check_document_dirty.lua lua/claudecode/tools/get_current_selection.lua lua/claudecode/tools/get_diagnostics.lua lua/claudecode/tools/get_latest_selection.lua lua/claudecode/tools/get_open_editors.lua lua/claudecode/tools/get_workspace_folders.lua lua/claudecode/tools/open_file.lua lua/claudecode/tools/save_document.lua tests/unit/server_spec.lua tests/unit/tools/get_diagnostics_spec.lua tests/unit/tools/open_file_spec.lua
• find lua -name '*.lua' -type f -print0 | xargs -0 -I{} mise exec -- luajit -e "assert(loadfile('{}'))"
• mise exec -- luacheck lua/ tests/ --no-unused-args --no-max-line-length
• mise exec -- busted -v <all test files> — 643 successes / 0 failures / 0 errors / 0 pending
• Dogfooded in a real Neovim terminal session with an IDE protocol scenario; captured screenshot and recording artifacts for review.

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