fix(lsp): omit params on shutdown and exit instead of sending null

[dergachoff]

Problem

Every mcpls shutdown against tsgo waits the full 3s CHILD_EXIT_GRACE and kills the child. Closes #403.

Root cause

LspClient::request/notify always emit Some(params), so the Value::Null passed by LspServer::shutdown serializes as "params": null. tsgo rejects that on shutdown with -32602 expected empty, got: null and ignores the following exit.

Fix

request and notify map Value::Null to None; the notification send path now serializes through JsonRpcNotification so skip_serializing_if drops the key. Non-null params are unchanged. Fixed in the client rather than the two call sites so any future params: void method is covered. Changelog entry under Unreleased.

Validation

• cargo +nightly fmt --check
• cargo clippy --all-targets --all-features --workspace -- -D warnings (stable 1.98.1)
• cargo nextest run --workspace: 812 passed
• New tests in client.rs::tests::void_params_wire: request with null params omits the key, notify with null params omits the key, notify with {} params keeps it
• Live: release build against tsgo 7.0.0-dev.20260611.2, shutdown returns result: null and the child exits in under 10ms with LSP server shut down successfully, versus the 3s kill fallback on v0.5.0

[Copilot]

🟢 Approval recommended

The change is narrowly scoped, aligns with JSON-RPC optional params semantics, and is covered by focused wire-level tests for both request and notification paths.

Pull request overview

This PR fixes JSON-RPC wire output for LSP params: void methods by omitting the params field entirely (instead of sending "params": null), preventing tsgo from rejecting shutdown/exit and allowing the LSP server process to exit promptly.

Changes:

• Map serde_json::Value::Null request/notification params to None so #[serde(skip_serializing_if = "Option::is_none")] drops the params key on the wire.
• Serialize notifications via JsonRpcNotification (instead of a manual json! object) to consistently apply skip_serializing_if.
• Add wire-level tests asserting that null params omit the key while {} params are preserved; add a changelog entry under Unreleased.

File summaries

File	Description
crates/mcpls-core/src/lsp/client.rs	Omits params when null for requests/notifications, switches notification serialization to JsonRpcNotification, and adds wire-format tests.
CHANGELOG.md	Documents the tsgo shutdown/exit fix under ## [Unreleased].

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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[bug-ops]

Thanks for the fix: clean root cause analysis, and appreciate routing it through the client layer.