feat(assert): match a cell by its OSC 8 link

[Ayman Bagabas (aymanbagabas)]

Makes OSC 8 links assertable, not just readable.

Stacked on #181. That PR records links on cells; this one adds the assertion surface. Review #181 first.

Why

#181 makes links observable through cells --json and snapshots, but the style query has no link field. So a test that wants to check where a link points has to shell out and parse JSON:

tui-test cells 0 0 4 1 --json | jq -r '.data.cells[0].link'

That is precisely what the style query exists to avoid. --fg, --underline-color and the rest are all first-class; the link was not.

What

link joins the other style properties, so it works everywhere they already do:

tui-test expect text "Docs" --link "https://example.com"
tui-test expect text "plain" --link ""

await term.getByStyle({ link: "https://example.com" }).expect();

await term.get_by_style(TextStyle(link="https://example.com")).expect()

Because it is an ordinary style property it also composes with the within, after and before locator stages for free.

An empty string is a requirement, not an absence

--link "" asks for a cell that links nowhere. That is how a test asserts OSC 8 ;; actually closed a link, rather than only that some other link is absent — a distinction that matters because a link running to the end of the row is exactly the bug #181 found in the xterm.js shim.

The Option already carries "the caller did not ask", which leaves the inner String free to mean something. Hence Option<String> rather than a bare String with a sentinel.

Verification

Unit tests cover the matcher: matching URL, non-matching URL, empty-link-vs-linked-cell in both directions, and a style with no link still matching a linked cell (so this does not silently narrow existing queries). A CLI test pins that --link "" survives parsing as an empty string rather than collapsing to None.

Checked against a live session too, since a parse test would not catch a wiring mistake:

assertion	exit
expect text "LINK" --link "https://example.com"	0
expect text "LINK" --link "https://wrong.example"	1
expect text "plain" --link ""	0
expect text "plain" --link "https://example.com"	1
expect text "LINK" --link ""	1

cargo test --workspace --features tui-test-rs/ghostty,tui-test-rs/rio,tui-test-rs/xtermjs gives 633 passed, 0 failed. cargo fmt --check and both clippy -D warnings invocations are clean.