Fix surround operations on selections ending at EOF - #16175

Summary

• accept surround selections whose half-open range ends exactly at EOF
• continue rejecting ranges that extend beyond the text
• cover EOF deletion through the existing core surround and transaction path

Closes #14923

Testing

• RED: cargo test -p helix-core surround::test::test_get_surround_pos_full_range_ending_at_eof_can_be_deleted -- --exact --nocapture failed with RangeExceedsText before the fix
• GREEN: cargo test -p helix-core surround::test (11 passed)
• cargo test -p helix-core (171 unit tests, 4 integration tests, and 5 doc tests passed; 3 doc tests ignored)
• HELIX_DISABLE_AUTO_GRAMMAR_BUILD=1 cargo test --workspace
• cargo fmt --all --check
• HELIX_DISABLE_AUTO_GRAMMAR_BUILD=1 cargo check
• HELIX_DISABLE_AUTO_GRAMMAR_BUILD=1 cargo clippy --workspace --all-targets -- -D warnings
• git diff --check

The Rust and Ruby tree-sitter grammars were fetched and built locally because the full helix-core syntax tests require them. HELIX_DISABLE_AUTO_GRAMMAR_BUILD=1 only disabled the expensive automatic fetch/build of all runtime grammars for the workspace commands; it did not skip Rust crates or tests. A default cargo check attempt was stopped after the generated gitcommit parser had been compiling at -O3 for over eight minutes; the command then passed with the automatic grammar build disabled.