fix ansi_html() when same style repeated

[trevorld]

After closing the span in html_cb_text(), reset state->old so the next text segment correctly detects it needs styling.

I manually wrote the unit tests, Claude Code proposed the fix but I manually moved their fix from the end of html_cb_text() to instead be inside clic__html_end() which I felt was a more natural location. Unsure if this is the "right" way to reset state...

closes #752

[trevorld]

• I observed a bunch of snapshot failures when I ran devtools::test() before I made any changes but observed no regressions particularly in test-ansi-html.R.

[trevorld]

• To be more thorough some additional tests of pasted together identical styles combined with various types of style nesting
• These extra tests all pass with this fix

test_that("nested styles: add then remove one while another remains active", {
  expect_equal(
    ansi_html("\033[1m\033[4mbold underline\033[24m just bold\033[22m"),
    "<span class=\"ansi ansi-bold ansi-underline\">bold underline</span><span class=\"ansi ansi-bold\"> just bold</span>"
  )
})

test_that("nested styles: foreground and background colors", {
  expect_equal(
    ansi_html("\033[31mred\033[42mred on green\033[49m red only\033[39m"),
    "<span class=\"ansi ansi-color-1\">red</span><span class=\"ansi ansi-color-1 ansi-bg-color-2\">red on green</span><span class=\"ansi ansi-color-1\"> red only</span>"
  )
})

test_that("consecutive strings with the same nested style pasted together", {
  x <- "\033[1m\033[31mred bold\033[39m\033[22m\033[1m\033[31mred bold\033[39m\033[22m"
  expect_equal(
    ansi_html(x),
    "<span class=\"ansi ansi-bold ansi-color-1\">red bold</span><span class=\"ansi ansi-bold ansi-color-1\">red bold</span>"
  )
  expect_equal(
    ansi_html(ansi_simplify(x)),
    "<span class=\"ansi ansi-bold ansi-color-1\">red boldred bold</span>"
  )
})

test_that("consecutive strings with the same inner style within an outer style", {
  x <- "\033[1m\033[31mR\033[39m\033[31mR\033[39m\033[22m"
  expect_equal(
    ansi_html(x),
    "<span class=\"ansi ansi-bold ansi-color-1\">R</span><span class=\"ansi ansi-bold ansi-color-1\">R</span>"
  )
  expect_equal(
    ansi_html(ansi_simplify(x)),
    "<span class=\"ansi ansi-bold ansi-color-1\">RR</span>"
  )
})