Problem: Argument type mismatches are excluded from EmmyLua checks.

Solution: Correct runtime annotations and calls, document local checker
workarounds, and enable the diagnostic.

AI-assisted

Enable EmmyLua's need-check-nil diagnostic and resolve nullable values
through more precise types, explicit invariants, and guards for missing
runtime state. Keep analyzer-specific workarounds local and update the
affected API documentation.

Exclude the bundled re.lua from analysis so its nil-check warnings do
not fail CI.

AI-assisted

Problem:
When vim.lsp.rpc.connect is used for an LSP and the connection fails
(due to ECONNREFUSED, for example), the LSP client is left
uninitialized. Subsequent calls to vim.lsp.start will reuse the
uninitialized client, but no new connection will be attempted.

Solution:
When vim.lsp.rpc.connect encounters a connection error, terminate the
client. This allows the user to do something in response to the failed
connection in the on_exit callback, and also makes it so subsequent
attempts to start the LSP will try to establish a new connection rather
than using the previous uninitialized client.

Co-authored-by: Jason Stenftenagel <jasonstenftenagel@gmail.com>

Problem: LuaLS struggles with the generics used in Nvim's runtime,
requiring broad diagnostic suppressions. Indexing is also slow.

Solution: Use EmmyLua for type checks in the build and CI. It offers
more sophisticated type checking, substantially better support for
generics, and much better flow analysis.

Correct the affected annotations. Use `@internal`, supported directly by
EmmyLua, instead of `@nodoc` for shared internal declarations, and
support it in the help parser.

AI-assisted

Problem:
`parse_ssh_config()` compares tables against a freshly allocated empty
table.
- In `parse_multiple_values()`, the guard which avoids flushing an empty
  accumulator never applies. Runs of separators and trailing whitespace
  push empty strings into the results, and `is_valid()` does not filter
  them. `Host  alpha   beta ` parses as `{ 'alpha', '', 'beta' }`.
- In `parse_value()`, the condition reduces to `chr == '"' and quoted`.
  `quoted` starts false and only that branch sets it, so it can never
  become true: quotes are never recognised and are inserted literally,
  and the unterminated-quote check is unreachable.

Solution:
Compare `#val` instead. Add a test for repeated and trailing separators.

AI-assisted