Fix URL hint matching and OSC 8 rule handling

[AdamsGH]

What changed

• Keep the configured hint rule for OSC 8 hyperlinks instead of replacing it with a synthetic Open rule.
• Apply the rule regex and post-processing setting to the hyperlink URI.
• Treat empty grid cells as whitespace during regex matching, so patterns such as [^\s]+ stop at the visible end of the line.

Why

OSC 8 hyperlinks currently bypass the configured rule action when clicked. Regex hint matching can also consume terminal padding because empty cells contain \0, which is not whitespace. Together these bugs produce the wrong action or match text for custom URL and path rules.

Testing

• cargo fmt --all -- --check
• cargo test -p rioterm --features wgpu
• Manually tested on Windows for several days with URL and path hint rules