fix: remove extra spaces in full function signatures

[lingyaochu]

Closes #23069

When rust-analyzer.completion.fullFunctionSignatures.enable is set to true, completion descriptions are generated by detail_full, which internally calls write_function() to produce the signature. For functions with more than four parameters, the generated signature is multi-line:

rust-analyzer/crates/hir/src/display.rs

Lines 195 to 210 in ece721d

	let too_long_param = data.params.len() > 4;
	f.write_char('(')?;
	if too_long_param {
	f.write_str("\n ")?;
	}
	let mut first = true;
	let mut skip_self = 0;
	if let Some(self_param) = func.self_param(db) {
	self_param.hir_fmt(f)?;
	first = false;
	skip_self = 1;
	}
	let comma = if too_long_param { ",\n " } else { ", " };

Take the compare_exchange() function as an example, the final generated signature for this function is like:

pub fn compare_exchange(
    &self,
    current: bool,
    new: bool,
    success: Ordering,
    failure: Ordering,
) -> Result<bool, bool>

Flattening this output in detail_full previously inserted a space right after ( and right before ), e.g. pub fn compare_exchange( &self, ... ).

This PR makes a small change to detail_full to skip the whitespace insertion around the parentheses.