[AryanRG]

Description

Addresses #606. When shutdown cancels a queued threaded request, its future callback removes an entry from _request_futures while lsp_shutdown is iterating that dictionary. This raises RuntimeError; the shutdown request receives an internal-error response and the protocol does not enter its shutdown state.

Iterate a snapshot of the futures instead. The regression keeps one worker occupied, queues two more requests, then checks that shutdown succeeds, the queued requests are cancelled, and callback cleanup completes.

Validation

• The new regression fails on the unchanged base on Python 3.9 and 3.13, and passes with this change.
• Full local suite on each version: 289 passed, 7 skipped.
• TCP end-to-end suite on Python 3.13: 74 passed. WebSocket end-to-end suite: 73 passed, 1 skipped.
• Ruff, Black, mypy, generated-code comparison, and whitespace checks passed.

Local runs were on macOS; hosted CI will cover other platforms and versions. uv lock --check --offline failed identically on base and this branch with local uv 0.13.1; the lockfile is unchanged. AI assistance was used to prepare and validate this change.