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[JavadAsadi]

Introduce LSP client infrastructure for Dart analysis server

Adds the foundational plumbing to run the Dart analysis server in LSP mode (dart language-server --protocol=lsp) as an alternative to the legacy JSON-based protocol.

Key additions:

• DartClient — interface abstracting over both protocols, allowing the rest of the plugin to remain unaware of which is active
• LegacyDartAnalysisClient — thin wrapper around the existing analysis server, implementing DartClient
• LspDartAnalysisClient — new LSP-based client; currently a stub with only diagnostic_getServerPort implemented
• LspClientConnectionManager — manages the LSP4J process lifecycle: startup, initialize/initialized handshake, status listener notifications, and clean shutdown
• DartExtendedLanguageServer — extends LSP4J's LanguageServer with @JsonRequest("dart/diagnosticServer") so the custom Dart extension method is properly typed and deserialized
• DartClientFactory — selects which client to instantiate based on a registry flag