Replace the Rust URL form-encoding callback bridge into C++ TextCodec
with a direct encoding_rs encoder.
This keeps percent-encode-after-encoding entirely in Rust and removes
liburl_rust's dependency on LibTextCodec.
It also happens to fix ISO-2022-JP URL encoding of literal U+FFFD.
The LibTextCodec reverse lookup treats generated 0xFFFD table holes as
real JIS0208 mappings, so literal U+FFFD skipped the encoder-error
path. encoding_rs treats U+FFFD as unmappable, so URL encoding emits
the required numeric character reference.
The Rust URL FFI used String::from_utf8_lossy() on raw bytes from C++.
That was not equivalent to the old C++ parser behavior for WTF-8 encoded
surrogates. For example a byte sequence such as ED A0 80 was treated as
three invalid UTF-8 bytes, producing three U+FFFD replacement
characters.
Instead use String::from_utf8_with_replacement_character and let Rust
borrow the input as &str.
