chore: remove DiagnosticRequestParams workaround when lsp-types fixes upstream null serialization

[bug-ops]

Description

DiagnosticRequestParams in crates/mcpls-core/src/bridge/translator.rs is a private workaround struct that duplicates lsp_types::DocumentDiagnosticParams. It exists because the upstream crate (lsp-types 0.97.0) lacks #[serde(skip_serializing_if = "Option::is_none")] on its identifier and previous_result_id fields, causing those fields to serialize as JSON null instead of being omitted.

tsgo's strict Go LSP decoder rejects null for these fields (InvalidParams: null value is not allowed for field "identifier"), which is what motivated the workaround introduced in #147.

Notably, the same lsp-types file already applies skip_serializing_if on DiagnosticOptions.identifier — the omission on DocumentDiagnosticParams appears to be an oversight.

Expected Behavior

Once gluon-lang/lsp-types adds #[serde(skip_serializing_if = "Option::is_none")] to DocumentDiagnosticParams.identifier and DocumentDiagnosticParams.previous_result_id, we can:

1. Bump the lsp-types dependency to the fixed version
2. Remove DiagnosticRequestParams and diagnostic_request_params() from translator.rs
3. Restore direct use of lsp_types::DocumentDiagnosticParams

Action

• Track the upstream fix in gluon-lang/lsp-types
• Bump lsp-types in Cargo.toml once the fix is released
• Remove DiagnosticRequestParams workaround from translator.rs
• Remove the test_diagnostic_request_params_omit_optional_null_fields test (covered by upstream type guarantees)

Environment

• lsp-types version: 0.97.0
• Workaround introduced in: fix(lsp): omit null diagnostic request fields #147