## Context

Closes #51330

When the cursor is at the very end of a file,
`find_specific_language_server_in_selection` in
`crates/editor/src/lsp_ext.rs` silently skips the selection because the
anchor's `buffer_id` is `None`. This causes `editor: switch source
header` (clangd) and rust-analyzer extension commands (`expand macro`,
`open docs`, `open playground`) to do nothing.

The fix falls back to the singleton buffer's ID when the anchor has no
`buffer_id`.

## How to Review

Single file change in `crates/editor/src/lsp_ext.rs`. The diff is small
— pre-compute the singleton buffer ID, then use it as fallback in the
`filter_map` closure. An integration test verifies the fix.

## Self-Review Checklist

- [x] Reviewed own diff for quality, security, and reliability
- [x] No unsafe blocks
- [x] Tests pass (639 editor tests, 0 failures)
- [x] Manual testing: switch source header works at beginning, middle,
and end of C++ file

## Test Plan

- [x] New integration test: `test_find_language_server_at_end_of_file` —
verifies `find_specific_language_server_in_selection` returns `Some` at
both beginning and end of file
- [x] Confirmed the test fails without the fix (assertion on "should
find language server at end of file")
- [x] Manual: open a C++ file with clangd, place cursor at very end, run
`switch source header` — now correctly opens the header
- [x] Manual: verify it still works at beginning and middle of file
(non-regression)
- [x] `cargo test -p editor` — 639 passed, 0 failed

Release Notes:

- Fixed `editor: switch source header` and other LSP extension commands
not working when the cursor is at the very end of a file.

---------

Co-authored-by: Florian Trehaut <florian.trehaut@hillcode.fr>
Co-authored-by: Kirill Bulatov <kirill@zed.dev>