fix: make watchdog more resilient against badly behaving clients

[mhuisi]

This PR addresses some non-critical but annoying issues that sometimes cause the language server to report an error:

• When using global search and replace in VS Code, the language client sends textDocument/didChange notifications for documents that it never told the server to open first. Instead of emitting an error and crashing the language server when this occurs, we now instead ignore the notification. Fixes search and replace across files crashes server #4435.
• When terminating the language server, VS Code sometimes still sends request to the language server even after emitting a shutdown request. The LSP spec explicitly forbids this, but instead of emitting an error when this occurs, we now error requests and ignore all other messages until receiving the final exit notification. Reported on Zulip several times over the years but never materialized as an issue, e.g. https://leanprover.zulipchat.com/#narrow/stream/270676-lean4/topic/Got.20.60shutdown.60.20request.2C.20expected.20an.20.60exit.60.20notification/near/441914289.
• Some language clients attempt to reply to the file watcher registration request before completing the LSP initialization dance. To fix this, we now only send this request after the initialization dance has completed. Fixes question: why wait for initialized is needed here? #3904.

[leanprover-community-mathlib4-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase c5120c1d0dd5a23f1432b6432e6d897659ebb63f --onto bedcbfcfeed429428c3e7f008b6984fc8c2b2b76. (2024-06-13 12:35:24)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 16cad2b45c6a77efe4dce850dcdbaafaa7c91fc3 --onto bedcbfcfeed429428c3e7f008b6984fc8c2b2b76. (2024-06-13 13:43:45)