netrc/lex: fix `lineno` not incremented in `read_line` (#19452)
## Summary
`Lex::read_line` iterates directly over `self.instream` instead of going
through `self.read_char`, which is the only place that increments
`self.lineno`. As a result, every newline consumed by `read_line` is
invisible to the line counter, causing parse errors reported after any
`macdef` block or bare-`#` comment to show an incorrect (too-low) line
number.
The fix is a one-line change: replace the `for ch in &mut self.instream`
loop with `while let Some(ch) = self.read_char()`. The parsed output is
identical; only the line-number bookkeeping is corrected.
The three call sites affected are:
- Discarding a bare `#` comment line in the top-level token loop
- Reading each line of a `macdef` macro body
- Discarding a bare `#` comment line in the follower-token loop
## Test Plan
Existing tests in `crates/uv-netrc/src/netrc.rs` continue to pass. The
bug manifests only in error-message line numbers, so no behavioural test
was previously catching it. A targeted regression test can be added if
the maintainers prefer — happy to include one on request.
