WASIp3 HTTP: Inconsistent errors when adding invalid field names

Consider this test for what the HTTP implementation does with invalid field names. First it tries a field name with a space in it, then it tries `"`. Both of these are [invalid](https://www.rfc-editor.org/rfc/rfc9110.html#section-5.6.2), but the first shows an error on `fields.delete` whereas the second does not:

```rust
extern crate wit_bindgen;

wit_bindgen::generate!({
inline: r"
package test:test;

world test {
include wasi:http/imports@0.3.0-rc-2025-09-16;
}
",
additional_derives: [PartialEq, Eq, Hash, Clone],
features:["clocks-timezone"],
generate_all
});

use wasi::http::types::Fields;
use wasi::http::types::HeaderError;

fn test_invalid_field_name(field: &str) {
println!("testing field: {}", field);
let fields = Fields::new();
assert!(!fields.has(field));
assert_eq!(fields.delete(field),
Err(HeaderError::InvalidSyntax));
}

fn test_invalid_field_names() {
test_invalid_field_name("field with space");
test_invalid_field_name("\"");
}

fn main() {
test_invalid_field_names();
}
```

Run it:

```
$ cargo +nightly build --target=wasm32-wasip2 --release && wasmtime -Wcomponent-model-async -Sp3,http target/wasm32-wasip2/release/field-invalid.wasm
Compiling test-wasm32-wasip3 v0.1.0 (/home/wingo/src/wasip3/wasi-testsuite/tests/rust/wasm32-wasip3)
Finished `release` profile [optimized] target(s) in 0.35s
testing field: field with space
testing field: "

thread 'main' (1) panicked at src/bin/field-invalid.rs:23:5:
assertion `left == right` failed
left: Ok(())
right: Err(HeaderError::InvalidSyntax)
```