Do not coalesce LSP responses as mutations

[heejaechang]

Summary

LSP responses are processed on the normal FIFO queue, but they were classified as state mutations. That classification also updates the queue's last_mutation marker, which tells earlier event handlers that a later mutation is waiting and that expensive work such as validation can be deferred.

This can lose diagnostics during startup:

1. The server receives textDocument/didOpen.
2. The client immediately replies to a server-initiated request such as workspace/configuration.
3. Because the response is marked as a mutation, dequeuing didOpen reports subsequent_mutation = true.
4. didOpen skips open-file validation under the assumption that the later mutation will make that work redundant.
5. A configuration response may be a no-op and does not necessarily trigger validation, so nothing compensates for the skipped work.

The server can then remain healthy and responsive while never publishing diagnostics for the opened file until some unrelated later event triggers validation.

This change classifies LspResponse with normal-priority query events instead. Responses still preserve FIFO ordering and are handled exactly as before, but they no longer claim that work triggered by preceding document mutations is redundant. This matches the queue's coalescing invariant: only events that can safely supersede prior mutation work should update last_mutation.

A deterministic regression enqueues didOpen followed by a configuration-shaped response and verifies that:

• didOpen is not reported as having a subsequent mutation, so validation is not suppressed.
• The response remains queued and is delivered in FIFO order.

Test Plan

• cargo test lsp::non_wasm::queue::tests --quiet
• python3 test.py --no-test --no-tensor-shapes --no-conformance --no-jsonschema

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D115504842. (Because this pull request was imported automatically, there will not be any future comments.)

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅