Accept the legacy boolean form of diagnostic tag support

Look at the ClientCapabilities decoding in the codebase, likely in a file handling LSP protocol types or initialization. The issue mentions gen-lsp-types and TagSupport::deserialize_compat. Find where tagSupport is parsed and modify it to accept a boolean (true/false) as a legacy form, mapping true to an empty value set and false to no support. Ensure the change doesn't break the object form. Write a test for initialize with both boolean forms and the object form, verifying the server starts successfully.

Context

Since #549, ClientCapabilities comes from gen-lsp-types, which types textDocument.publishDiagnostics.tagSupport as the ClientDiagnosticsTagOptions object defined by the current specification. lsp-types 0.95 decoded it through TagSupport::deserialize_compat, which also accepted the legacy boolean form: true as tag support with an empty value set, and false as no tag support.

A client that still sends the boolean now has initialize rejected with InvalidParams (invalid type: boolean 'true', expected struct ClientDiagnosticsTagOptions), for both true and false, so the language server cannot start. The protocol actor's Session::negotiate silently falls back to default capabilities in the same case, but that does not help once the workspace rejects initialize.

The server does not read tagSupport. It is unknown which clients still send the boolean form; lsp-types kept the compatibility path because some did.

Goal

A client that sends the legacy boolean tagSupport can initialize the server, as it could with lsp-types 0.95.

Scope

• Accept or ignore the boolean form rather than failing capability decoding. Decoding only the capabilities the server uses would also resolve this, and could share a solution with #551.
• Avoid the protocol actor and the workspace interpreting the same capabilities differently.

Tests

• initialize with tagSupport: true and with tagSupport: false succeeds and returns the usual server capabilities.
• The object form keeps working.