fix(lsp): omit null diagnostic request fields

[dergachoff]

Problem

get_diagnostics fails against tsgo --lsp --stdio because mcpls sends textDocument/diagnostic params with absent optional fields serialized as JSON null:

{
  "identifier": null,
  "previousResultId": null
}

tsgo's Go LSP decoder rejects this with:

InvalidParams: ... within "/identifier": null value is not allowed for field "identifier"

Fix

Use a small diagnostics request params type that omits absent identifier and previousResultId fields instead of serializing them as null.

Validation

• cargo +nightly fmt --check
• cargo +1.95 test -p mcpls-core
• cargo +1.95 clippy -p mcpls-core --all-targets --all-features -- -D warnings
• Fresh local MCP probe against tsgo: get_diagnostics returns diagnostics instead of the identifier: null protocol error