Problem: Repeated default fields and field-by-field assertions obscure
the values that each functional test is meant to check.

Solution: Add eq_partial() to check selected record fields while
preserving list length and order. Use local helpers to fill fixed
expected defaults where tests need full equality. Keep explicit absence
checks and test-specific values at the call sites.

AI-assisted

Problem:
`vim.lsp.buf.format()` does not support a timeout for asynchronous
requests.

Solution:
Honor an explicit timeouts per client so formatting can continue.
Do not permit a default value for async format requests.

Problem:
With the addition of the `:help al` text object, you can now easily
format the whole buffer with `gqal`. However, `vim.lsp.formatexpr` only
uses `textDocument/rangeFormatting` which some language servers (like
gopls) don't support.

Solution:
- Fall back to `textDocument/formatting` if the whole buffer is being formatted
  and the server doesn't support `textDocument/rangeFormatting`.
  - In theory, these two methods should return the same response if the whole
    buffer is being formatted, but I preserved the existing behaviour of
    prioritising `textDocument/rangeFormatting` in case that does not hold (i.e.
    LS bug).
- Also: `vim.lsp.formatexpr` had no tests at all, so actually add tests for it.

Problem:
The magic globals `it`, `describe`, etc., are more trouble than they are
worth.

- Hooking into `after_each` requires `getfenv()` hacks.
- They confuse luals/emmylua, because the top-level `.luarc.json` isn't
  merged with `test/.luarc.json` (apparently a luals limitation?)
- They totally defeat discoverability because the user just has to
  "know" about the various magic symbols.

So they harm DX, which means they serve no purpose at all.

Solution:
- Expose the test API from `testutil`, so tests can call `t.it()`,
  `t.describe()`, etc., in the conventional way.
- Drop `getfenv()` hacks.
- Drop the `setfenv()` injection in `load_chunk`.
- Drop `test/_meta.lua`.

Problem:
`vim.lsp.buf.format()` accepts ranges using nvim indexing, where an
end column of -1 means end of line. LSP ranges cannot use that,
which is confusing for things like range formatting.

Solution:
Resolve -1 end columns to the line length before converting the range to
LSP positions.

Problem:
The fromRanges field of the result of callHierarchy/outgoingCalls is
documented as being relative to the caller. Using
vim.lsp.buf.outgoing_calls() opened the qflist with an entry with the
callee's filename, but the caller's line number.

Solution:
Open the qflist with the callers file (the bufnr from the request),
rather than the callees (the uri from the resulting CallHierarchyItem)

Problem:
Cursor-position `vim.lsp.buf.code_action()` requests include all diagnostics on the current line, so unrelated same-line diagnostics affect the returned actions.

Solution:
Filter same-line diagnostics to the cursor position for cursor-position requests.

Problem:
`test/functional/plugin/lsp_spec.lua` had grown into a large catch-all file that mixed core LSP client lifecycle coverage, `vim.lsp.buf.*` behavior, and `vim.lsp.util.*` behavior in one place.

Solution:
Split the large tests into more focused test files without changing test coverage or intended behavior.

After this change, `lsp_spec.lua` is more focused on core LSP client/config/dynamic-registration behavior.