Add LSP integration tests

[geoffjay]

Summary

Add integration tests for the nemo-lsp server: verify completions for component names, attribute names, and enum values; hover documentation; diagnostics for invalid configs; and go-to-definition for templates and data sources.

Context

The LSP operates over JSON-RPC via stdin/stdout. Integration tests should:

• Start the LSP binary as a child process
• Send initialize, textDocument/didOpen, and other LSP messages
• Verify the JSON-RPC responses contain expected completion items, diagnostics, hover content, etc.
• Use sample Nemo XML files that exercise all completion and diagnostic scenarios

Acceptance Criteria

• Test harness starts nemo-lsp binary and communicates over stdin/stdout JSON-RPC
• Test: textDocument/completion returns component element names when inside <layout>
• Test: textDocument/completion returns attribute names for a known component (button, label)
• Test: textDocument/completion returns enum values for constrained string attributes (variant, size)
• Test: textDocument/hover returns property documentation with type and default
• Test: textDocument/diagnostic reports missing required attributes
• Test: textDocument/diagnostic reports unknown component types
• Test: textDocument/definition navigates to template definitions
• Test: textDocument/definition navigates to data source definitions
• Test: completion for template="..." returns template names from current file
• Test: completion for source="..." in <binding> returns data source names
• Sample XML files in crates/nemo-lsp/tests/fixtures/ covering edge cases
• Tests runnable via cargo test -p nemo-lsp

Relevant Files

• crates/nemo-lsp/ — the LSP server
• crates/nemo-config/src/xml_parser.rs — XML parsing used by LSP
• crates/nemo-config/src/validator.rs — validation used by diagnostics

Stack Base

• Stack on: main
• Blocked by: Create nemo-lsp language server #62 (nemo-lsp language server) — tests are written against the completed LSP