Scala: fix parsing fragility around case bodies and try/catch keywords

[knutwannheden]

Motivation

Several "find next token" lookups in the Scala parser used a fixed
+50 / +200 character substring window. When intervening comments
push the target past the window, the parser silently misses the token
and produces non-idempotent print output. This surfaced as parse
failures on real Scala files containing things like
case _: Throwable => // comment (where a long line comment pushes
the closing } past the lookahead window).

A ; between cases on the same line (case a => x; case b => y) was
also being dropped on print because there was no marker preserving the
explicit separator.

Summary

• Empty catch case bodies with trailing line comments now round-trip
  (case _: Throwable => // comment\n}) — replaced the cursor + 50
  window with positionOfNext(\"}\") which skips over comments.
• Explicit ; separators between same-line match cases are preserved
  via a Semicolon marker on the JRightPadded wrapping the case.
  Detected by checking whether the next non-whitespace token after the
  body's content end (computed from the AST's Block.expr/stats
  spans, not the cursor — which Dotty's case body span often overshoots)
  is ;.
• catch and finally keyword searches no longer use a fixed lookahead
  window — works correctly when long comments sit between the try body
  and catch, or between catch and finally.
• Tightens up the parser internals: uses Space.format(source, start, end)
  instead of Space.format(source.substring(start, end)) throughout
  (avoids 70+ intermediate String allocations on the hot path), and
  uses Tree.randomId() instead of UUID.randomUUID() at marker
  construction sites.

Test plan

• New round-trip tests in MatchTest for ;-separated cases and
  various case body shapes (./gradlew :rewrite-scala:test --tests MatchTest)
• New round-trip tests in TryTest for empty case bodies with line
  comments, long block comments before catch, and long block
  comments before finally
• Full Scala test suite passes (./gradlew :rewrite-scala:test)